FBI agents are reportedly scrutinising images of a man running from the scene of the Boston Marathon bombings.

The shots, taken by amateur photographer Benjamin Thorndike from his nearby office window, show the young man behaving in stark contrast to those surrounding him.

Thorndike stresses he is not labelling the man a suspect, telling The Boston Globe: “For all I know, this is a guy who was watching the race who got burned.”

boston marathon bombing running man

Amateur photographer Benjamin Thorndike took these photographs from his office window

However he points out to ABC News: “I was struck by this individual.

“Everyone else in the photo is on the ground, bent over, holding their ears, appearing to be in shock, immobile, but this one individual is exactly the opposite.

“He’s sprinting. His clothes are in tatters. They were melted off, burning off his body, so for all I know he was panicked and fleeing.”

boston marathon bombing running man

Thorndike has stressed he does not know if the man is a suspect

The pictures have been handed over to the FBI, who have spoken to Thorndike several times, Boston CBS reports.

It adds: “The FBI did not reveal anything to Thorndike about the investigation or the running man.”

The FBI are scouring thousands of photographs and video of the scene, where two explosions killed three and injured more than 180.

Also of interest are before and after shots of an orange and grey bag pictured on the route where the second bomb detonated.

boston bomb

The FBI is examining these images of a backpack on the Boston Marathon route where the second bomb detonated

The before shot shows the bag next to a mailbox along a barricade, while the after shot, taken after the explosion, shows no sign of the bag.

Images of one of the bombs have been released showing the remains of a black backpack which contained the device.

The FBI says it has evidence it was contained in a pressure cooker filled with nails and ball bearings.

boston bomb

This torn black backpack is believed to have contained one of the bombs

boston bomb
This shows the remains of a pressure cooker the FBI say was part of one of the bombs

Theories also abound about an image of a man standing on the roof of a nearby building, which was posted on Twitter.

Three people, including eight-year-old Martin Richard who was hoping to see his runner father cross the finishing line, lost their lives in the blasts. Krystal Campbell, 29, and Lu Lingzi, 23 were also killed.

man on roof

A Twitter user caught this image just as the explosion happened towards the finishing line.

While pressure cooker bombs are more commonly used in Afghanistan, Pakistan, India and Nepal; they have been seen before in domestic terror attacks, including the 2010 attempted bombing in Times Square.

No one has claimed responsibility for the bombings and no arrests have been made.
On Wednesday Boston police denied reports made by some US media outlets that a suspect had been arrested.

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  • In this photo provided by The Daily Free Press and Kenshin Okubo, people react to an explosion at the 2013 Boston Marathon in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria of the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/The Daily Free Press, Kenshin Okubo) MANDATORY CREDIT

  • Medical workers aid injured people at the finish line of the 2013 Boston Marathon following an explosion Monday, April 15, 2013 in Boston. Two bombs exploded near the finish line of the marathon on Monday, killing at least two people, injuring at least 22 others and sending authorities rushing to aid wounded spectators. (AP Photo/Charles Krupa)

  • Boston police clear an area following an explosion near the finish line of the 2013 Boston Marathon in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria of the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/Charles Krupa)

  • Medical personnel work outside the medical tent in the aftermath of two blasts which exploded near the finish line of the Boston Marathon in Boston, Monday, April 15, 2013. (AP Photo/Elise Amendola)

  • An unidentified Boston Marathon runner, center, is reunited with loved ones near Copley Square following an explosion in Boston Monday, April 15, 2013. (AP Photo/Winslow Townson)

  • Medical workers aid an injured woman at the finish line of the 2013 Boston Marathon following two explosions there, Monday, April 15, 2013 in Boston. Two bombs exploded near the finish of the Boston Marathon on Monday, killing at least two people, injuring at least 23 others and sending authorities rushing to aid wounded spectators. (AP Photo/Charles Krupa)

  • Workers aid injured people at the finish line of the 2013 Boston Marathon following an explosion in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria of the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/The Boston Herald, Stuart Cahill) MANDATORY CREDIT

  • In this photo provided by The Daily Free Press and Kenshin Okubo, people react to an explosion at the 2013 Boston Marathon in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria of the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/The Daily Free Press, Kenshin Okubo) MANDATORY CREDIT

  • In this photo provided by The Daily Free Press and Kenshin Okubo, people react to an explosion at the 2013 Boston Marathon in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria of the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/The Daily Free Press, Kenshin Okubo) MANDATORY CREDIT

  • In this photo provided by The Daily Free Press and Kenshin Okubo, people assist an injured after an explosion at the 2013 Boston Marathon in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria of the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/The Daily Free Press, Kenshin Okubo) MANDATORY CREDIT

  • Medical workers aid an injured woman at the scene of a bomb blast near the finish line of the 2013 Boston Marathon following an explosion in Boston, Monday, April 15, 2013. Two bombs exploded near the finish line of the Boston Marathon on Monday, killing at least two people and injuring at scores of others (AP Photo/Charles Krupa)

  • Medical workers aid injured people at the finish line of the 2013 Boston Marathon in Boston, Monday, April 15, 2013. Two bombs exploded near the finish line of the Boston Marathon on Monday, killing at least two people and injuring dozens of others (AP Photo/Charles Krupa)

  • Runners who were diverted from the race course walk on the Commonwealth Mall two blocks from the site of an explosion at the finish line of the Boston Marathon in Boston, Monday, April 15, 2013. Two bombs exploded near the finish line of the Boston Marathon on Monday, killing at least two people and injuring at least 23 others. (AP Photo/Michael Dwyer)

  • Una mujer llora afectada por una explosión mientras caminan cerca de la línea de meta de la Maratón de Boston el lunes 15 de abril de 2013. Dos bombas estallaron el lunes causando la muerte de dos personas y heridas a otras 50, informó la Policía de Boston. (AP Foto/Josh Reynolds)

  • An armed FBI agent passes a Boston police officer following an explosion at the finish line of the 2013 Boston Marathon in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria of the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/Josh Reynolds)

  • A firefighter tends to an injured man following an explosion near the finish line of the Boston Marathon in Boston, Monday, April 15, 2013. (AP Photo/Michael Dwyer)

  • One of the blast sites on Boylston Street near the finish line of the 2013 Boston Marathon is investigated by a person in a protective suit in the wake of two blasts in Boston, Monday, April 15, 2013. (AP Photo/Elise Amendola)

  • Firefighters tend to a man following an explosion at the finish line of the Boston Marathon in Boston, Monday, April 15, 2013. Two bombs exploded at the Boston Marathon finish line Monday killing at least two people injuring dozens. (AP Photo/Michael Dwyer)

  • In this photo provided by The Daily Free Press and Kenshin Okubo, people react to an explosion at the 2013 Boston Marathon in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria of the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/The Daily Free Press, Kenshin Okubo) MANDATORY CREDIT

  • Two people walk down the sidewalk as Boston police clear the area in Copley Square in the aftermath of two blasts which exploded near the finish line of the Boston Marathon in Boston Monday, April 15, 2013. The explosions Monday killed at least two people and injured dozens. (AP Photo/Elise Amendola)

  • This photo provided by Bruce Mendelsohn shows the scene after two explosions occurred during the 2013 Boston Marathon in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria of the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/ Bruce Mendelsohn) MANDATORY CREDIT

  • Medical workers respond following an explosion at the 2013 Boston Marathon in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria of the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/The Boston Globe, David L Ryan) MANDATORY CREDIT

  • Boston police direct runners who were diverted from the race course following an explosion at the finish line of the Boston Marathon in Boston, Monday, April 15, 2013. (AP Photo/Michael Dwyer)

  • Medical workers aid injured people at the finish line of the 2013 Boston Marathon following an explosion in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria of the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/The Boston Globe, David L Ryan) MANDATORY CREDIT

  • Medical workers aid injured people at the 2013 Boston Marathon following an explosion in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria of the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/The Boston Globe, David L Ryan) MANDATORY CREDIT

  • Blood from victims covers the sidewalk on Boylston Street, at the site of an explosion during the 2013 Boston Marathon in Boston, Monday, April 15, 2013. At the right foreground is a folding chair with the design of an American flag on the cover. Two explosions shattered the euphoria of the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/Charles Krupa)

  • Chris Darmody, right, holds his wife Sue in Boston, Monday, April 15, 2013. Chris says he was waiting for Sue when an explosion detonated near his location at the finish line of the Boston Marathon. The couple were later reunited after all runners were diverted from the course. (AP Photo/Michael Dwyer)

  • Emergency responders comfort a woman on a stretcher who was injured in a bomb blast near the finish line of the Boston Marathon Monday, April 15, 2013 in Boston. Two bombs exploded in the packed streets near the finish line of the marathon on Monday, killing at least two people and injuring more than 80, authorities said. (AP Photo/Jeremy Pavia)

  • Emergency responders aid a woman on a stretcher who was injured in a bomb blast near the finish line of the Boston Marathon Monday, April 15, 2013 in Boston. Two bombs exploded in the packed streets near the finish line of the marathon on Monday, killing at least two people and injuring more than 80, authorities said. (AP Photo/Jeremy Pavia)

  • Emergency responders comfort a woman on a stretcher who was injured in a bomb blast near the finish line of the Boston Marathon Monday, April 15, 2013 in Boston. Two bombs exploded in the packed streets near the finish line of the marathon on Monday, killing at least two people and injuring more than 80, authorities said. (AP Photo/Jeremy Pavia)

  • Rescue personnel aid injured people near the finish line of the 2013 Boston Marathon following explosions in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria of the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/The Boston Herald, Stuart Cahill)

  • Rescue personnel aid injured people near the finish line of the 2013 Boston Marathon following explosions in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria of the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/The Boston Herald, Stuart Cahill)

  • Injured people and debris lie on the sidewalk near the Boston Marathon finish line following an explosion in Boston, Monday, April 15, 2013. (AP Photo/MetroWest Daily News, Ken McGagh) MANDATORY CREDIT

  • As Boston Marathon runners walk by, SWAT team members stand guard near the finish line in Boston Monday, April 15, 2013. Two explosions shattered the euphoria of the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. ( (AP Photo/Winslow Townson)

  • People react to an explosion at the 2013 Boston Marathon in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria of the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/The Boston Herald, Stuart Cahill) BOSTON GLOBE OUT; METRO BOSTON OUT; MAGS OUT;

  • Investigators shine flashlights at one of the blast sites on Boylston Street near the finish line of the 2013 Boston Marathon in the wake of two blasts in Boston Monday, April 15, 2013. (AP Photo/Elise Amendola)

  • In this image from video provided by Ryan Hoyme, the second explosion can be seen in the distance as smoke from the first explosion surrounds spectators exiting the stands during the Boston Marathon in Boston, Monday, April 15, 2013. (AP Photo/Ryan Hoyme)

  • knocked down runner

    Bill Iffrig, 78, lies on the ground as police officers react to a second explosion at the finish line of the Boston Marathon in Boston, Monday, April 15, 2013. Iffrig, of Lake Stevens, Wash., was running his third Boston Marathon and near the finish line when he was knocked down by one of two bomb blasts. (AP Photo/The Boston Globe, John Tlumacki)

  • A Boston Marathon runner leaves the course crying near Copley Square following an explosion at the finish line in Boston on Monday, April 15, 2013. (AP Photo/Winslow Townson)

  • People react as an explosion goes off near the finish line of the 2013 Boston Marathon in Boston, Monday, April 15, 2013. Two explosions went off at the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/The Boston Globe, David L Ryan) MANDATORY CREDIT

  • Multiple People Injured After Explosions Near Finish Line at Boston Marathon

    BOSTON, MA - APRIL 15: A runner embraces another woman on the marathon route near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Alex Trautwig/Getty Images)

  • Multiple People Injured After Explosions Near Finish Line at Boston Marathon

    BOSTON, MA - APRIL 15: A runner reacts near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Alex Trautwig/Getty Images)

  • Multiple People Injured After Explosions Near Finish Line at Boston Marathon

    BOSTON, MA - APRIL 15: A runner embraces another woman near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Alex Trautwig/Getty Images)

  • Multiple People Injured After Explosions Near Finish Line at Boston Marathon

    BOSTON, MA - APRIL 15: A woman is loaded into an ambulance after he was injured by one of two bombs exploded during the 117th Boston Marathon near Copley Square on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Jim Rogash/Getty Images)

  • Multiple People Injured After Explosions Near Finish Line at Boston Marathon

    BOSTON, MA - APRIL 15: A woman is loaded into an ambulance after he was injured by one of two bombs exploded during the 117th Boston Marathon near Copley Square on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Jim Rogash/Getty Images)

  • Multiple People Injured After Explosions Near Finish Line at Boston Marathon

    BOSTON, MA - APRIL 15: A woman, with blood on fingers, is loaded into an ambulance after being injured after two bombs exploded on the marathon route on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Jim Rogash/Getty Images)

  • Multiple People Injured After Explosions Near Finish Line at Boston Marathon

    BOSTON, MA - APRIL 15: A woman looks at the blood on her hands as she is loaded into an ambulance after being injured after two bombs exploded on the marathon route on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Jim Rogash/Getty Images)

  • Multiple People Injured After Explosions Near Finish Line at Boston Marathon

    BOSTON, MA - APRIL 15: Two blood stained feet of a man hang outside an ambulance outside a medical tent located near the finish of the 117th Boston Marathon after two bombs exploded on the marathon route on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Jim Rogash/Getty Images)

  • Boston police ask people to leave the area in Copley Plaza in the aftermath of two blasts which exploded near the finish line of the Boston Marathon in Boston Monday, April 15, 2013. (AP Photo/Elise Amendola)

  • Paramédicos asisten a un hombre herido después de una explosión en Boston el Lunes 15 de Abril de 2013. Dos bombas estallaron cerca de la línea de meta del maratón de Boston, causaron dos muertos y 22 heridos, dijeron la policía y los organizadores de la competición. (AP Foto/The Boston Globe, David L. Ryan)

  • Uniformed Division Secret Service officers stand watch on the sidewalk in front of the White House looking north from Pennsylvania Avenue during heightened security following the explosions in Boston, Monday, April 15, 2013 in Washington. (AP Photo/Charles Dharapak)


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  • Neighbors come to pay their respects at the Richard house in the Dorchester neighborhood of Boston, Tuesday, April 16, 2013. Martin Richard, 8, was killed in Monday's bombings at the finish line of the Boston Marathon. (AP Photo/Michael Dwyer)

  • Tammy Lynch, right, comforts her daughter Kaytlyn, 8, after leaving flowers and some balloons at the Richard house in the Dorchester neighborhood of Boston, Tuesday, April 16, 2013. Kaytlyn was paying her respects to her friend, 8-year old Martin Richard who was killed in Monday's bombings at the finish line of the Boston Marathon. (AP Photo/Michael Dwyer)

  • Barack Obama

    President Barack Obama speaks in the Brady Press Briefing Room of the White House in Washington, Tuesday, April 16, 2013, following the explosions at the Boston Marathon. (AP Photo/Charles Dharapak)

  • Jodie Brown of England carries her luggage past a guarded checkpoint near the finish line of the Boston Marathon on her way to catch a plane in Boston Tuesday, April 16, 2013. (AP Photo/Winslow Townson)

  • First time Boston Marathon runner Amber Lazo of Miami pulls her luggage past a guarded checkpoint near the finish line of the Boston Marathon on her way to catch a flight in Boston Tuesday, April 16, 2013. Her sister, Jocelyn Siedle pushes thirteen-month-old Nina Lazo in a baby stroller behind her. (AP Photo/Winslow Townson)

  • Responders search the area around the Boston Public Library near the finish line of the Boston Marathon with a ladder truck in Boston Tuesday, April 16, 2013. Two bombs blew up seconds apart Monday at the finish line of one of the world's most storied races, tearing off limbs and leaving the streets spattered with blood and strewn with broken glass. At least three people were killed, including an 8-year-old boy, and more than 170 were wounded. (AP Photo/Winslow Townson)

  • Jodie Brown of England carries her luggage past a guarded checkpoint near the finish line of the Boston Marathon on her way to catch a plane in Boston Tuesday, April 16, 2013. Two bombs blew up seconds apart Monday at the finish line of one of the world's most storied races, tearing off limbs and leaving the streets spattered with blood and strewn with broken glass. Three people were killed, including an 8-year-old boy, and more than 170 were wounded. (AP Photo/Winslow Townson)

  • An Amtrak Police officer and his sniffer dog monitor travelers at the 30th Street Station Tuesday, April 16, 2013, in Philadelphia. The deadly explosions at the Boston Marathon on Monday reverberated on both sides of the Atlantic as cities from Los Angeles to London saw an increase in security. (AP Photo/Matt Rourke)

  • The flag flies at half staff in front of the Neighborhood House Charter School in the Dorchester neighborhood of Boston, Tuesday, April 16, 2013. Boston Marathon bombing victim, 8-year old Martin Richard attended the school. (AP Photo/Michael Dwyer)

  • Barack Obama

    President Barack Obama finishes speaking in the Brady Press Briefing Room of the White House in Washington, Tuesday, April 16, 2013, about the Boston Marathon explosions.(AP Photo/Susan Walsh)

  • Nicholas Yanni

    Nicholas Yanni, 32, of Boston, speaks to reporters at Tufts Medical Center in Boston, Tuesday, April 16, 2013. Yanni, and his wife, Lee Ann Yanni, 31, not pictured, were among the 14 patients injured in the bombing at the finish of the Boston Marathon who were treated at Tufts. (AP Photo/Josh Reynolds)

  • A U.S. Capitol police officer monitors the area as an American flag flies at half-staff on Capitol Hill in Washington, Tuesday, April 16, 2013, after Monday's Boston Marathon explosions. (AP Photo/Jose Luis Magana)

  • Nicholas Yanni

    Nicholas Yanni, 32, of Boston, speaks to reporters at Tufts Medical Center in Boston, Tuesday, April 16, 2013. Yanni, and his wife, Lee Ann Yanni, 31, were among the 14 patients injured in the bombing at the finish of the Boston Marathon who were treated at Tufts. (AP Photo/Josh Reynolds)

  • Security personnel stand in front of a PATH train station at the World Trade Center site in New York, Tuesday, April 16, 2013. Police armed with rifles and extra patrol cars were stationed around the city Tuesday as New York remained in a heightened state of alert until more is known about the Monday bombings at the Boston Marathon. (AP Photo/Seth Wenig)

  • Los Angeles International Airport Police Officer, Daniel Keehne works security with partner Axa Tuesday April 16, 2013 at the airport in Los Angeles. Security measures have been increased in light of Monday's bombings at the Boston Marathon. (AP Photo/Nick Ut)

  • "Pray for Martin" is written in chalk at a park near the home of Martin Richard in the Dorchester neighborhood of Boston,Tuesday, April 16, 2013. 8-year old Martin was killed in the bombing at the finish line of the Boston Marathon on Monday. (AP Photo/Michael Dwyer)

  • Bureau of Alcohol, Tobacco and Firearms, (ATF) agents and Boston police detectives gather near the finish line of the Boston Marathon in Boston, Tuesday, April 16, 2013. The bombs that ripped through the crowd at the Boston Marathon, killing at least three people and wounding more than 170, were fashioned out of pressure cookers and packed with shards of metal, nails and ball bearings to inflict maximum carnage, a person briefed on the investigation said Tuesday. (AP Photo/Winslow Townson)

  • FBI agents gather near the finish line of the Boston Marathon in Boston Tuesday, April 16, 2013. The bombs that ripped through the crowd at the Boston Marathon, killing at least three people and wounding more than 170, were fashioned out of pressure cookers and packed with shards of metal, nails and ball bearings to inflict maximum carnage, a person briefed on the investigation said Tuesday. (AP Photo/Winslow Townson)

  • Bureau of Alcohol, Tobacco and Firearms, (ATF) agents and Boston police detectives gather near the finish line of the Boston Marathon in Boston, Tuesday, April 16, 2013. The bombs that ripped through the crowd at the Boston Marathon, killing at least three people and wounding more than 170, were fashioned out of pressure cookers and packed with shards of metal, nails and ball bearings to inflict maximum carnage, a person briefed on the investigation said Tuesday. (AP Photo/Winslow Townson)

  • Two men in haz-mat suits investigate the scene of the first bombing on Boylston Street in Boston Tuesday, April 16, 2013 near the finish line of the 2013 Boston Marathon, a day after two blasts nearby killed three and injured over 170 people. (AP Photo/Elise Amendola)

  • Investigators in haz-mat suits examine the scene of the second bombing on Boylston Street in Boston Tuesday, April 16, 2013 near the finish line of the 2013 Boston Marathon, a day after two blasts killed three and injured over 170 people. (AP Photo/Elise Amendola)

  • Investigators in haz-mat suits examine the scene of the second bombing on Boylston Street in Boston Tuesday, April 16, 2013 near the finish line of the 2013 Boston Marathon, a day after two blasts killed at least three and injured over 170 people. (AP Photo/Elise Amendola)

  • Flowers sit at a police barrier near the finish line of the Boston Marathon in Boston Tuesday, April 16, 2013. Explosions at the finish of the Marathon Monday Killed at least 3 people and injured over 170. (AP Photo/Winslow Townson)

  • James Skoufis, Patricia Fahy, Luis Sepulveda

    New York Assembly members James Skoufis, D-Woodbury, left, Patricia Fahy, D-Albany, center, and Luis Sepulveda, D-Bronx, join other legislators in a moment of silence for victims of the explosions at Monday's Boston Marathon, in the Assembly Chamber at the Capitol in Albany, N.Y., on Tuesday, April 16, 2013. (AP Photo/Mike Groll)

  • A jetliner flies over flags flying at half-staff at the Washington Monument in Washington, Tuesday, April 16, 2013, after President Obama ordered flags to be lowered on federal building to honor the loss of life from the explosions at the Boston Marathon. (AP Photo/J. David Ake)

  • A bystander takes a photo from a window as Seattle Police Dept. robots are used to remove an item from a suspicious backpack in downtown Seattle, Tuesday, April 16, 2013. The backpack was found to be harmless, but in light of Monday's explosions at the Boston Marathon, authorities in Seattle and other cities increased security patrols and heightened the level of alertness for any possible threat to public safety. (AP Photo/Ted S. Warren)

  • Ed Coco, Jill Coco

    Boston marathon runner Ed Coco, left, stands with his wife Jill Coco, both of Atlanta, after the couple arrived at Hartsfield-Jackson airport from Boston, Tuesday, April 16, 2013, in Atlanta. "The venue was just the most beautiful venue I've ever been at." said Ed Coco. "It's bittersweet. It's sad that it had to end that way. We're happy to be home. It's very nerve wrecking." The deadly explosions near the finish line at the Boston Marathon on Monday killed nearly three and injured more than 140 people. (AP Photo/David Goldman)

  • Two men in haz-mat suits investigate the scene near the first bombing on Boylston Street in Boston Tuesday, April 16, 2013 near the finish line of the 2013 Boston Marathon, a day after two blasts nearby killed at least three and injured over 170 people. (AP Photo/Elise Amendola)

  • Seattle Police Dept. vehicles block off a street in while remote controlled vehicles are used to examine a suspicious backpack in downtown Seattle, Tuesday, April 16, 2013. The backpack was found to be harmless, but in light of Monday's explosions at the Boston Marathon, authorities in Seattle and other cities increased security patrols and heightened the level of alertness for any possible threat to public safety. (AP Photo/Ted S. Warren)

  • A Seattle Police officer looks on as Seattle Police Dept. robots are used to open a suspicious backpack in downtown Seattle, Tuesday, April 16, 2013. The backpack was found to be harmless, but in light of Monday's explosions at the Boston Marathon, authorities in Seattle and other cities increased security patrols and heightened the level of alertness for any possible threat to public safety. (AP Photo/Ted S. Warren)

  • Birds fly away as a Seattle Police Dept. robotic vehicle is driven back to its truck after it was used to examine a suspicious backpack in downtown Seattle, Tuesday, April 16, 2013. The backpack was found to be harmless, but in light of Monday's explosions at the Boston Marathon, authorities in Seattle and other cities increased security patrols and heightened the level of alertness for any possible threat to public safety. (AP Photo/Ted S. Warren)

  • An official carries away a backpack that contained a hair dryer and other items after Seattle Police Dept. robots at left were used used to check out the suspicious backpack in downtown Seattle, Tuesday, April 16, 2013. The backpack was found to be harmless, but in light of Monday's explosions at the Boston Marathon, authorities in Seattle and other cities increased security patrols and heightened the level of alertness for any possible threat to public safety. (AP Photo/Ted S. Warren)

  • Neighbors hug outside the home of the Richard family in the Dorchester neighborhood of Boston, Tuesday, April 16, 2013. Martin Richard, 8, was killed in Mondays bombing at the finish line of the Boston Marathon. (AP Photo/Michael Dwyer)

  • Peace is written on the sidewalk in front of the Richard house in the Dorchester neighborhood of Boston, Tuesday, April 16, 2013. Martin Richard, 8, was killed in Monday's bombing at the finish line of the Boston Marathon. (AP Photo/Michael Dwyer)

  • Two women place flowers on the doorstep of the Richard house in the Dorchester neighborhood of Boston, Tuesday, April 16, 2013. Martin Richard, 8, was killed in the Mondays bombings at the finish line of the Boston Marathon. The boy’s mother, Denise, and 6-year-old sister, Jane, were badly injured. (AP Photo/Michael Dwyer)

  • US-ATTACKS-BOSTON

    Boston police officers near a Humvee April 16, 2013 in Boston, Massachusetts, in the aftermath of two explosions that struck near the finish line of the Boston Marathon April 15. A massive probe was underway Tuesday after two bombs struck the Boston Marathon, killing at least three and wounding more than 100. Monday's blasts near the finishing line raised fears of a terrorist attack more than a decade after nearly 3,000 people were killed in suicide airliner strikes on New York, Washington and Pennsylvania on September 11, 2001. US President Barack Obama went on national television to warn against 'jumping to conclusions' but a senior White House official, speaking on condition of anonymity, said such an attack was 'clearly an act of terror.' AFP PHOTO Stan HONDA (Photo credit should read STAN HONDA/AFP/Getty Images)

  • Boston Deals With Aftermath Of Marathon Explosions

    BOSTON - APRIL 16: Police officers stand near the Boston Marathon finish line from Clarendon and Boylston Street on April 16, 2013 in Boston, Massachusetts. Security is especially tight in the city of Boston after two explosions went off near the finish of the Marathon, killing three people and injuring at least 141 others. (Photo by Darren McCollester/Getty Images)

  • Boston Deals With Aftermath Of Marathon Explosions

    BOSTON - APRIL 16: A sign on a door makes notice of being closed at Trident Booksellers Cafe on Newbury Street shows closed on April 16, 2013 in Boston, Massachusetts. Security is especially tight in the city of Boston after two explosions went off near the finish of the Marathon, killing three people and injuring at least 141 others. (Photo by Darren McCollester/Getty Images)

  • Boston Deals With Aftermath Of Marathon Explosions

    BOSTON, MA - APRIL 16: Claire Schaeffer Dufy walks with a sign she made to support her runner husband, Scott, near the scene of a twin bombing at the Boston Marathon on April 16, 2013 in Boston, Massachusetts. The twin bombings, which occurred near the marathon finish line, resulted in the deaths of three people while hospitalizing at least 128. The bombings at the 116-year-old Boston race, resulted in heightened security across the nation with cancellations of many professional sporting events as authorities search for a motive to the violence. (Photo by Spencer Platt/Getty Images)

  • Workers find bags containing runners' personal effects as they return them to their owner near the finish line of the Boston Marathon, after explosions killed three and injured more than 140 in Boston, Tuesday, April 16, 2013. The bombs that blew up seconds apart at the finish line of one of the world's most storied races left the streets spattered with blood and glass, and gaping questions of who chose to attack at the Boston Marathon and why. (AP Photo/Charles Krupa)

  • Boston Deals With Aftermath Of Marathon Explosions

    BOSTON, MA - APRIL 16: People congregate at a security gate near the scene of a twin bombing at the Boston Marathon on April 16, 2013 in Boston, Massachusetts. The twin bombings, which occurred near the marathon finish line, resulted in the deaths of three people while hospitalizing at least 128. The bombings at the 116-year-old Boston race, resulted in heightened security across the nation with cancellations of many professional sporting events as authorities search for a motive to the violence. (Photo by Spencer Platt/Getty Images)

  • Boston Deals With Aftermath Of Marathon Explosions

    BOSTON, MA - APRIL 16: A Boston police officer stands guard near the scene of a twin bombing at the Boston Marathon on April 16, 2013 in Boston, Massachusetts. The twin bombings, which occurred near the marathon finish line, resulted in the deaths of three people while hospitalizing at least 128. The bombings at the 116-year-old Boston race, resulted in heightened security across the nation with cancellations of many professional sporting events as authorities search for a motive to the violence. (Photo by Spencer Platt/Getty Images)

  • Boston Deals With Aftermath Of Marathon Explosions

    BOSTON, MA - APRIL 16: A napkin and broken salt and pepper shakers are seen after customers fled an outdoor restaurant near the scene of a twin bombing at the Boston Marathon on April 16, 2013 in Boston, Massachusetts. The twin bombings, which occurred near the marathon finish line, resulted in the deaths of three people while hospitalizing at least 128. The bombings at the 116-year-old Boston race, resulted in heightened security across the nation with cancellations of many professional sporting events as authorities search for a motive to the violence. (Photo by Spencer Platt/Getty Images)

  • Boston Deals With Aftermath Of Marathon Explosions

    BOSTON, MA - APRIL 16: The unfinished meals of fleeing customers are left on tables at an outdoor restaurant near the scene of a twin bombing at the Boston Marathon on April 16, 2013 in Boston, Massachusetts. The twin bombings, which occurred near the marathon finish line, resulted in the deaths of three people while hospitalizing at least 128. The bombings at the 116-year-old Boston race, resulted in heightened security across the nation with cancellations of many professional sporting events as authorities search for a motive to the violence. (Photo by Spencer Platt/Getty Images)

  • Boston Deals With Aftermath Of Marathon Explosions

    BOSTON, MA - APRIL 16: A heavily armed Boston police officer stands blocks away from the scene of a twin bombing at the Boston Marathon on April 16, 2013 in Boston, Massachusetts. The twin bombings, which occurred near the marathon finish line, resulted in the deaths of three people while hospitalizing at least 128. The bombings at the 116-year-old Boston race, resulted in heightened security across the nation with cancellations of many professional sporting events as authorities search for a motive to the violence. (Photo by Spencer Platt/Getty Images)

  • Boston Deals With Aftermath Of Marathon Explosions

    BOSTON, MA - APRIL 16: Unused thermal blankets for marathon participants are piled near the scene of a twin bombing at the Boston Marathon on April 16, 2013 in Boston, Massachusetts. The twin bombings, which occurred near the marathon finish line, resulted in the deaths of three people while hospitalizing at least 128. The bombings at the 116-year-old Boston race, resulted in heightened security across the nation with cancellations of many professional sporting events as authorities search for a motive to the violence. (Photo by Spencer Platt/Getty Images)

  • Boston Deals With Aftermath Of Marathon Explosions

    BOSTON, MA - APRIL 16: A Boston police officer stands near the scene of a twin bombing at the Boston Marathon on April 16, 2013 in Boston, Massachusetts. The twin bombings, which occurred near the marathon finish line, resulted in the deaths of three people while hospitalizing at least 128. The bombings at the 116-year-old Boston race, resulted in heightened security across the nation with cancellations of many professional sporting events as authorities search for a motive to the violence. (Photo by Spencer Platt/Getty Images)

  • Boston Deals With Aftermath Of Marathon Explosions

    BOSTON, MA - APRIL 16: A runner stands at a security gate near the scene of a twin bombing at the Boston Marathon on April 16, 2013 in Boston, Massachusetts. The twin bombings, which occurred near the marathon finish line, resulted in the deaths of three people while hospitalizing at least 128. The bombings at the 116-year-old Boston race, resulted in heightened security across the nation with cancellations of many professional sporting events as authorities search for a motive to the violence. (Photo by Spencer Platt/Getty Images)

  • Boston Deals With Aftermath Of Marathon Explosions

    BOSTON, MA - APRIL 16: A Boston police officer stands near the scene of a twin bombing at the Boston Marathon on April 16, 2013 in Boston, Massachusetts. The twin bombings, which occurred near the marathon finish line, resulted in the deaths of three people while hospitalizing at least 128. The bombings at the 116-year-old Boston race, resulted in heightened security across the nation with cancellations of many professional sporting events as authorities search for a motive to the violence. (Photo by Spencer Platt/Getty Images)

  • Boston Deals With Aftermath Of Marathon Explosions

    BOSTON, MA - APRIL 16: National Guard soldiers walk near the scene of a twin bombing at the Boston Marathon on April 16, 2013 in Boston, Massachusetts. The twin bombings, which occurred near the marathon finish line, resulted in the deaths of three people while hospitalizing at least 128. The bombings at the 116-year-old Boston race, resulted in heightened security across the nation with cancellations of many professional sporting events as authorities search for a motive to the violence. (Photo by Spencer Platt/Getty Images)

  • Boston Deals With Aftermath Of Marathon Explosions

    BOSTON, MA - APRIL 16: A discarded runners bib is viewed near the scene of a twin bombing at the Boston Marathon on April 16, 2013 in Boston, Massachusetts. The twin bombings, which occurred near the marathon finish line, resulted in the deaths of three people while hospitalizing at least 128. The bombings at the 116-year-old Boston race, resulted in heightened security across the nation with cancellations of many professional sporting events as authorities search for a motive to the violence. (Photo by Spencer Platt/Getty Images)