Boston Marathon bombing suspect Dzhokhar Tsarnaev remains at large after the death of his brother Tamerlan Tsarnaev, who is also suspected of carrying out the attacks.

Speculation is flooding social networks as more details come to light. Here is the chain of events following the Boston Marathon bombings.

April 15

Two explosions occur within 15 seconds of each other around four hours into the race. The attack may have been timed for maximum carnage, as the four-hour mark is typically a crowded time near the finish line.

Initial reports say two people have been killed but the death toll later goes up to three. Hundreds more are injured, many seriously.

April 16

US President Barack Obama says those responsible for the attacks will feel the "full weight of justice".
The FBI says no one has claimed responsibility for the bombings.

April 17

There are reports that a suspect has been taken into custody in connection but this is later denied by the authorities.

April 18

President Obama declares "every one of us stands with you" as he addresses an interfaith service to remember the victims at Boston's Cathedral of the Holy Cross.

Investigators are said to be focused on hunting a man seen on surveillance footage from a department store near the finish line, dropping off a bag and walking away from the site of the second of the two explosions.

5.30pm (10.30pm UK time) - The FBI releases CCTV footage and stills of two suspects from surveillance cameras near the explosion sites filmed shortly before the blasts.

FBI agent Richard DesLauriers asks for the public's help in identifying the men who are both wearing baseball caps and carrying backpacks. "We consider them to be armed and extremely dangerous," Mr DesLauriers said, warning that they should not be approached.

10.30pm (3.30am UK time) - A campus police officer is shot and killed at Massachusetts Institute of Technology (MIT) in Cambridge, near Boston, after responding to a report of a disturbance.

Shortly afterwards, police get a report of a carjacking nearby. The two men are suspected of killing the MIT police officer, then stealing the car at gunpoint and later releasing its driver unharmed. The suspects threw explosives from the car as police followed it to the Boston suburb of Watertown.

April 19

1am (6am UK time) - Witnesses report hearing multiple gunshots and explosions in Watertown. Residents are advised to keep their doors locked and not let anyone in. TV footage shows armed officers surrounding a suspect lying on the ground.

Police later say one of the two suspects in the MIT officer shooting is dead but the other, who is tied to the Boston marathon bombing, remains at large.

Public transport is suspended in the Boston area as the hunt for the remaining suspect continues.

A new photo of him on the loose is released showing him in a grey hoodie sweatshirt at a 7-Eleven store in Cambridge.

The two suspects are said to be from a Russian region near Chechnya and have lived in the US for at least one year.

The surviving Boston bomb suspect is identified as Dzhokhar A Tsarnaev, 19, of Cambridge, Massachusetts.

SEE ALSO

Loading Slideshow...
  • A police officer signals for people to get back as they conducts a search for a suspect in the Boston Marathon bombings, Friday, April 19, 2013, in Watertown, Mass. Two suspects in the Boston Marathon bombing killed an MIT police officer, injured a transit officer in a firefight and threw explosive devices at police during a getaway attempt in a long night of violence that left one of them dead and another still at large Friday, authorities said as the manhunt intensified for a young man described as a dangerous terrorist.(AP Photo/Matt Rourke)

  • Police run with their weapons drawn as they conduct a search for a suspect in the Boston Marathon bombings, Friday, April 19, 2013, in Watertown, Mass. Two suspects in the Boston Marathon bombing killed an MIT police officer, injured a transit officer in a firefight and threw explosive devices at police during a getaway attempt in a long night of violence that left one of them dead and another still at large Friday, authorities said as the manhunt intensified for a young man described as a dangerous terrorist. (AP Photo/Matt Rourke)

  • Police cover as they surround a home while searching for a suspect in the Boston Marathon bombings in Watertown, Mass., Friday, April 19, 2013. The two suspects in the Boston Marathon bombing killed an MIT police officer and hurled explosives at police in a car chase and gun battle overnight that left one of them dead and his brother on the loose, authorities said Friday. (AP Photo/Charles Krupa)

  • A neighbor is escorted to safety as police surround a home while searching for a suspect in the Boston Marathon bombings in Watertown, Mass., Friday, April 19, 2013. The two suspects in the Boston Marathon bombing killed an MIT police officer and hurled explosives at police in a car chase and gun battle overnight that left one of them dead and his brother on the loose, authorities said Friday. (AP Photo/Charles Krupa)

  • Police surround a home while searching for a suspect in the Boston Marathon bombings in Watertown, Mass., Friday, April 19, 2013. The two suspects in the Boston Marathon bombing killed an MIT police officer and hurled explosives at police in a car chase and gun battle overnight that left one of them dead and his brother on the loose, authorities said Friday. (AP Photo/Charles Krupa)

  • Police surround a home while searching for a suspect in the Boston Marathon bombings in Watertown, Mass., Friday, April 19, 2013. The two suspects in the Boston Marathon bombing killed an MIT police officer and hurled explosives at police in a car chase and gun battle overnight that left one of them dead and his brother on the loose, authorities said Friday. (AP Photo/Charles Krupa)

  • A man walks out of his home in a bath robe as police surround a home while searching for a suspect in the Boston Marathon bombings in Watertown, Mass., Friday, April 19, 2013. The two suspects in the Boston Marathon bombing killed an MIT police officer and hurled explosives at police in a car chase and gun battle overnight that left one of them dead and his brother on the loose, authorities said Friday.(AP Photo/Charles Krupa)

  • Law enforcement officials stand guard while searching for an individual believed to be involved in the Boston Marathon explosions in Watertown, Mass., Friday, April 19, 2013. The two suspects in the Boston Marathon bombing killed an MIT police officer and hurled explosives at police in a car chase and gun battle overnight that left one of them dead and his brother on the loose, authorities said Friday. (AP Photo/Julio Cortez)

  • Residents are seen near a window, top, as an official wearing tactical gear stands guard while searching for an individual believed to be involved in the Boston Marathon explosions in Watertown, Mass., Friday, April 19, 2013. Two suspects in the Boston Marathon bombing killed an MIT police officer, injured a transit officer in a firefight and threw explosive devices at police during a getaway attempt in a long night of violence that left one of them dead and another still at large Friday, authorities said as the manhunt intensified for a young man described as a dangerous terrorist. (AP Photo/Julio Cortez)

  • Boston Marathon Explosions

    A police officer points his weapon at a residence as he conducts a search for a suspect in the Boston Marathon bombings, Friday, April 19, 2013, in Watertown, Mass. The two suspects in the Boston Marathon bombing killed an MIT police officer and hurled explosives at police in a car chase and gun battle overnight that left one of them dead and his brother on the loose, authorities said Friday as thousands of officers swarmed the streets in a manhunt that all but paralyzed the Boston area. (AP Photo/Matt Rourke)

  • Boston Marathon Explosions

    Police officers take cover as they conduct a search for a suspect in the Boston Marathon bombings, Friday, April 19, 2013, in Watertown, Mass. The two suspects in the Boston Marathon bombing killed an MIT police officer and hurled explosives at police in a car chase and gun battle overnight that left one of them dead and his brother on the loose, authorities said Friday as thousands of officers swarmed the streets in a manhunt that all but paralyzed the Boston area. (AP Photo/Matt Rourke)

  • A barefooted woman runs for cover as police surround a home while searching for a suspect in the Boston Marathon bombings in Watertown, Mass., Friday, April 19, 2013. Two suspects in the Boston Marathon bombing killed an MIT police officer, injured a transit officer in a firefight and threw explosive devices at police during a getaway attempt in a long night of violence that left one of them dead and another still at large Friday, authorities said as the manhunt intensified for a young man described as a dangerous terrorist. (AP Photo/Charles Krupa)

  • An official wearing tactical gear, left, looks on as others enter an apartment building in Watertown, Mass., Friday, April 19, 2013. Two suspects in the Boston Marathon bombing killed an MIT police officer, injured a transit officer in a firefight and threw explosive devices at police during a getaway attempt in a long night of violence that left one of them dead and another still at large Friday, authorities said as the manhunt intensified for a young man described as a dangerous terrorist. (AP Photo/Julio Cortez)

  • Heavily armed police continue to patrol the neighborhoods of Watertown, Mass. Friday, April 19, 2013, as they continue a massive search for one of two suspects in the Boston Marathon bombing. A second suspect died in the early morning hours after an encounter with law enforcement. (AP Photo/Craig Ruttle)

  • Tactical teams drive through a neighborhood while searching for a suspect in the Boston Marathon bombings in Watertown, Mass., Friday, April 19, 2013. Two suspects in the Boston Marathon bombing killed an MIT police officer, injured a transit officer in a firefight and threw explosive devices at police during a getaway attempt in a long night of violence that left one of them dead and another still at large Friday, authorities said as the manhunt intensified for a young man described as a dangerous terrorist. (AP Photo/Charles Krupa)

  • Officials wearing tactical gear search an apartment building in Watertown, Mass., Friday, April 19, 2013. Two suspects in the Boston Marathon bombing killed an MIT police officer, injured a transit officer in a firefight and threw explosive devices at police during a getaway attempt in a long night of violence that left one of them dead and another still at large Friday, authorities said as the manhunt intensified for a young man described as a dangerous terrorist. (AP Photo/Julio Cortez)

  • A police officer runs with his weapon drawn as he conduct a search for a suspect in the Boston Marathon bombings, Friday, April 19, 2013, in Watertown, Mass. Two suspects in the Boston Marathon bombing killed an MIT police officer, injured a transit officer in a firefight and threw explosive devices at police during a getaway attempt in a long night of violence that left one of them dead and another still at large Friday, authorities said as the manhunt intensified for a young man described as a dangerous terrorist. (AP Photo/Matt Rourke)

  • Officials wearing tactical gear search an apartment building in Watertown, Mass., Friday, April 19, 2013. Two suspects in the Boston Marathon bombing killed an MIT police officer, injured a transit officer in a firefight and threw explosive devices at police during a getaway attempt in a long night of violence that left one of them dead and another still at large Friday, authorities said as the manhunt intensified for a young man described as a dangerous terrorist. (AP Photo/Julio Cortez)

  • Tactical teams drive through a neighborhood while searching for a suspect in the Boston Marathon bombings in Watertown, Mass., Friday, April 19, 2013. Two suspects in the Boston Marathon bombing killed an MIT police officer, injured a transit officer in a firefight and threw explosive devices at police during a getaway attempt in a long night of violence that left one of them dead and another still at large Friday, authorities said as the manhunt intensified for a young man described as a dangerous terrorist. (AP Photo/Charles Krupa)

  • An official wearing tactical gear, right, looks on as others enter an apartment building in Watertown, Mass., Friday, April 19, 2013. Two suspects in the Boston Marathon bombing killed an MIT police officer, injured a transit officer in a firefight and threw explosive devices at police during a getaway attempt in a long night of violence that left one of them dead and another still at large Friday, authorities said as the manhunt intensified for a young man described as a dangerous terrorist. (AP Photo/Julio Cortez)

  • Police run with their weapons drawn as they conduct a search for a suspect in the Boston Marathon bombings, Friday, April 19, 2013, in Watertown, Mass. Two suspects in the Boston Marathon bombing killed an MIT police officer, injured a transit officer in a firefight and threw explosive devices at police during a getaway attempt in a long night of violence that left one of them dead and another still at large Friday, authorities said as the manhunt intensified for a young man described as a dangerous terrorist. (AP Photo/Matt Rourke)

  • Police officers take cover as they conduct a search for a suspect in the Boston Marathon bombings, Friday, April 19, 2013, in Watertown, Mass. Two suspects in the Boston Marathon bombing killed an MIT police officer, injured a transit officer in a firefight and threw explosive devices at police during a getaway attempt in a long night of violence that left one of them dead and another still at large Friday, authorities said as the manhunt intensified for a young man described as a dangerous terrorist. (AP Photo/Matt Rourke)

  • A police officer runs with his weapon drawn as he conduct a search for a suspect in the Boston Marathon bombings, Friday, April 19, 2013, in Watertown, Mass. Two suspects in the Boston Marathon bombing killed an MIT police officer, injured a transit officer in a firefight and threw explosive devices at police during a getaway attempt in a long night of violence that left one of them dead and another still at large Friday, authorities said as the manhunt intensified for a young man described as a dangerous terrorist. (AP Photo/Matt Rourke)

  • A police officer signals for people to get back as they conducts a search for a suspect in the Boston Marathon bombings, Friday, April 19, 2013, in Watertown, Mass. Two suspects in the Boston Marathon bombing killed an MIT police officer, injured a transit officer in a firefight and threw explosive devices at police during a getaway attempt in a long night of violence that left one of them dead and another still at large Friday, authorities said as the manhunt intensified for a young man described as a dangerous terrorist.(AP Photo/Matt Rourke)

  • Tactical teams exit an apartment building while searching for a suspect in the Boston Marathon bombings in Watertown, Mass., Friday, April 19, 2013. Two suspects in the Boston Marathon bombing killed an MIT police officer, injured a transit officer in a firefight and threw explosive devices at police during a getaway attempt in a long night of violence that left one of them dead and another still at large Friday, authorities said as the manhunt intensified for a young man described as a dangerous terrorist. (AP Photo/Charles Krupa)

  • Heavily armed police enter a building in Watertown, Mass. Friday, April 19, 2013, as law enforcement officials continue a massive search for one of two suspects in the Boston Marathon bombing. A second suspect died in the early morning hours after an encounter with law enforcement. (AP Photo/Craig Ruttle)

  • A police officer evacuates a shoeless man holding a child as members of law enforcement conduct a search for a suspect in the Boston Marathon bombings, Friday, April 19, 2013, in Watertown, Mass. Two suspects in the Boston Marathon bombing killed an MIT police officer, injured a transit officer in a firefight and threw explosive devices at police during a getaway attempt in a long night of violence that left one of them dead and another still at large Friday, authorities said as the manhunt intensified for a young man described as a dangerous terrorist. (AP Photo/Matt Rourke)

  • A police officer runs with his weapon drawn as he conduct a search for a suspect in the Boston Marathon bombings, Friday, April 19, 2013, in Watertown, Mass. Two suspects in the Boston Marathon bombing killed an MIT police officer, injured a transit officer in a firefight and threw explosive devices at police during a getaway attempt in a long night of violence that left one of them dead and another still at large Friday, authorities said as the manhunt intensified for a young man described as a dangerous terrorist.(AP Photo/Matt Rourke)

  • Boston Marathon Explosions

    A police officer points his weapon at a residence as he conducts a search for a suspect in the Boston Marathon bombings, Friday, April 19, 2013, in Watertown, Mass. The two suspects in the Boston Marathon bombing killed an MIT police officer and hurled explosives at police in a car chase and gun battle overnight that left one of them dead and his brother on the loose, authorities said Friday as thousands of officers swarmed the streets in a manhunt that all but paralyzed the Boston area. (AP Photo/Matt Rourke)

  • Heavily armed police continue to patrol the neighborhoods of Watertown, Mass. Friday, April 19, 2013, as they continue a massive search for one of two suspects in the Boston Marathon bombing. A second suspect died in the early morning hours after an encounter with law enforcement. (AP Photo/Craig Ruttle)

  • Boston Marathon Explosions

    A police officer with is weapon drawn conducts a search for a suspect in the Boston Marathon bombings, Friday, April 19, 2013, in Watertown, Mass. The two suspects in the Boston Marathon bombing killed an MIT police officer and hurled explosives at police in a car chase and gun battle overnight that left one of them dead and his brother on the loose, authorities said Friday as thousands of officers swarmed the streets in a manhunt that all but paralyzed the Boston area. (AP Photo/Matt Rourke)

  • Boston Marathon Explosions

    A police officer and a Federal Bureau of Investigation agent run as they conduct a search for a suspect in the Boston Marathon bombings, Friday, April 19, 2013, in Watertown, Mass. The two suspects in the Boston Marathon bombing killed an MIT police officer and hurled explosives at police in a car chase and gun battle overnight that left one of them dead and his brother on the loose, authorities said Friday as thousands of officers swarmed the streets in a manhunt that all but paralyzed the Boston area. (AP Photo/Matt Rourke)

  • Officials wearing tactical gear stand near an armored vehicle as they search an apartment building for one of two suspects in the Boston Marathon bombing, in Watertown, Mass., Friday, April 19, 2013. Two suspects in the Boston Marathon bombing killed an MIT police officer, injured a transit officer in a firefight and threw explosive devices at police during a getaway attempt in a long night of violence that left one of them dead and another still at large Friday, authorities said as the manhunt intensified for a young man described as a dangerous terrorist. (AP Photo/Julio Cortez)

  • Investigators continue to search for one of two suspects in the Boston Marathon bombing, in Watertown, Mass., Friday, April 19, 2013. Two suspects in the Boston Marathon bombing killed an MIT police officer, injured a transit officer in a firefight and threw explosive devices at police during a getaway attempt in a long night of violence that left one of them dead and another still at large Friday, authorities said as the manhunt intensified for a young man described as a dangerous terrorist. (AP Photo/Matt Rourke)

  • Investigators continue to search for one of two suspects in the Boston Marathon bombing, in Watertown, Mass., Friday, April 19, 2013. Two suspects in the Boston Marathon bombing killed an MIT police officer, injured a transit officer in a firefight and threw explosive devices at police during a getaway attempt in a long night of violence that left one of them dead and another still at large Friday, authorities said as the manhunt intensified for a young man described as a dangerous terrorist. (AP Photo/Matt Rourke)

  • Members of the FBI, State Police, Boston Police, Cambridge Police, and other law enforcement agencies, survey the perimeter near the home of suspect #2 on Norfolk Street April 19, 2013 in Cambridge, Massachusetts. A Massachusetts Institute of Technology (MIT) campus police officer was shot and killed late Thursday night at the school's campus in Cambridge. A short time later, police reported exchanging gunfire with alleged carjackers in Watertown, a city near Cambridge. One suspect has been killed during the car chase and police are seeking another, believed to be the same person (known as Suspect Two) wanted in connection with the deadly bombing at the Boston Marathon earlier this week. Police have confirmed that the dead assailant is Suspect One from the recently released marathon bombing photographs. (Photo by Jared Wickerham/Getty Images)

  • Residents are seen near a window, top, as an official wearing tactical gear stands guard while searching for an individual believed to be involved in the Boston Marathon explosions in Watertown, Mass., Friday, April 19, 2013. Two suspects in the Boston Marathon bombing killed an MIT police officer, injured a transit officer in a firefight and threw explosive devices at police during a getaway attempt in a long night of violence that left one of them dead and another still at large Friday, authorities said as the manhunt intensified for a young man described as a dangerous terrorist. (AP Photo/Julio Cortez)

  • Residents and their pets are evacuated as members of the FBI, State Police, Boston Police, Cambridge Police, and other law enforcement agencies, survey the perimeter near the home of suspect #2 on Norfolk Street April 19, 2013 in Cambridge, Massachusetts. A Massachusetts Institute of Technology (MIT) campus police officer was shot and killed late Thursday night at the school's campus in Cambridge. A short time later, police reported exchanging gunfire with alleged carjackers in Watertown, a city near Cambridge. One suspect has been killed during the car chase and police are seeking another, believed to be the same person (known as Suspect Two) wanted in connection with the deadly bombing at the Boston Marathon earlier this week. Police have confirmed that the dead assailant is Suspect One from the recently released marathon bombing photographs. (Photo by Jared Wickerham/Getty Images)

  • Police continue to patrol the neighborhoods of Watertown, Mass. Friday, April 19, 2013, as a massive search continued for one of two suspects in the Boston Marathon bombing. A second suspect died in the early morning hours after an encounter with law enforcement. (AP Photo/Craig Ruttle)

  • Boston Marathon Explosions

    Police presence grows in the neighborhoods of Watertown, Mass., Friday, April 19, 2013, as a massive search involving hundreds of heavily armed law enforcement officials continues for one of two suspects in the Boston Marathon bombing. A second suspect died in the early morning hours in a car chase and gun battle overnight that left one of them dead and his brother on the loose, authorities said Friday. (AP Photo/Craig Ruttle)

  • Boston Marathon Explosions

    Police in tactical gear arrive on an armored police vehicle as they surround an apartment building while looking for a suspect in the Boston Marathon bombings in Watertown, Mass., Friday, April 19, 2013. The bombs that blew up seconds apart near the finish line of the Boston Marathon left the streets spattered with blood and glass, and gaping questions of who chose to attack and why. (AP Photo/Charles Krupa)

  • Boston Marathon Explosions

    Police walk through a neighborhood in Watertown, Mass., while searching for a suspect in the Boston Marathon bombings, Friday, April 19, 2013. The bombs that blew up seconds apart near the finish line of the Boston Marathon left the streets spattered with blood and glass, and gaping questions of who chose to attack and why. (AP Photo/Charles Krupa)

  • A bus with law enforcement agents travels through Watertown, Mass., Friday, April 19, 2013. All residents of Boston were ordered to stay in their homes Friday morning as the search for the surviving suspect in the marathon bombings continued after a long night of violence that left another suspect dead. (AP Photo/Julio Cortez)

  • Boston Marathon Explosions

    A police officer in tactical gear conducts a search for a suspect in the Boston Marathon bombings, Friday, April 19, 2013, in Watertown, Mass. The bombs that blew up seconds apart near the finish line of the Boston Marathon left the streets spattered with blood and glass, and gaping questions of who chose to attack and why. (AP Photo/Matt Rourke)

  • Police in tactical gear conduct a search for a suspect in the Boston Marathon bombings, Friday, April 19, 2013, in Watertown, Mass. The bombs that blew up seconds apart near the finish line of the Boston Marathon left the streets spattered with blood and glass, and gaping questions of who chose to attack and why. (AP Photo/Matt Rourke)

  • Boston Marathon Explosions

    Police in tactical gear conduct a search for a suspect in the Boston Marathon bombings, Friday, April 19, 2013, in Watertown, Mass. The bombs that blew up seconds apart near the finish line of the Boston Marathon left the streets spattered with blood and glass, and gaping questions of who chose to attack and why. (AP Photo/Matt Rourke)

  • Boston Marathon Explosions

    A Department of Homeland Security officer, right, prepares to enter an apartment building while looking for a suspect in the Boston Marathon bombings in Watertown, Mass., Friday, April 19, 2013. The bombs that blew up seconds apart near the finish line of the Boston Marathon left the streets spattered with blood and glass, and gaping questions of who chose to attack and why. (AP Photo/Charles Krupa)

  • Boston Marathon Explosions

    With his weapon drawn, an official inspects an apartment building in Watertown, Mass., Friday, April 19, 2013. Dozens of officers and National Guard members are in Watertown, where gunfire and explosions were heard early Friday morning. (AP Photo/Julio Cortez)

  • Police in tactical gear conduct a search for a suspect in the Boston Marathon bombings, Friday, April 19, 2013, in Watertown, Mass. All residents of Boston were ordered to stay in their homes Friday morning as the search for the surviving suspect in the marathon bombings continued after a long night of violence that left another suspect dead. (AP Photo/Matt Rourke)

  • Residents look out a window as officials inspect an apartment building in Watertown, Mass., Friday, April 19, 2013. Two suspects in the Boston Marathon bombing killed an MIT police officer, injured a transit officer in a firefight and threw explosive devices at police during their getaway attempt in a long night of violence that left one of them dead and another still at large Friday. (AP Photo/Julio Cortez)

  • Boston Marathon Explosions

    Police in tactical gear arrive on an armored police vehicle as they surround an apartment building while looking for a suspect in the Boston Marathon bombings in Watertown, Mass., Friday, April 19, 2013. The bombs that blew up seconds apart near the finish line of the Boston Marathon left the streets spattered with blood and glass, and gaping questions of who chose to attack and why. (AP Photo/Charles Krupa)

Loading Slideshow...
  • Suspects 1 and 2

  • This frame grab from a video released by the FBI on Thursday, April 18, 2013, shows what the FBI are calling suspect number 1, front, in black cap, and suspect number 2, in white cap, back right, walking near each other through the crowd in Boston on Monday, April 15, 2013, before the explosions at the Boston Marathon. (AP Photo/FBI)

  • Suspects 1 and 2

  • Suspect 1

  • Suspect 1

  • Suspect 1

  • Suspect 2

  • This image from video released Thursday, April 19, 2013 by the FBI shows one of two suspects sought in connection with the Boston Marathon bombings. (AP Photo/FBI)

  • Suspect 2

  • This combination of Associated Press file images released by the FBI on Thursday, April 18, 2013, show two images taken from surveillance video of what the FBI are calling suspect number 2, left, in white cap,and suspect number 1, right, in black cap, as they walk near each other through the crowd in Boston on Monday, April 15, 2013, before the explosions at the Boston Marathon. (AP Photo/FBI, File)

  • This frame grab from a video released by the FBI on Thursday, April 18, 2013, shows what the FBI are calling suspect number 1, black cap in lower right, walking in front of suspect number 2, not seen, through the crowd in Boston on Monday, April 15, 2013, before the explosions at the Boston Marathon. (AP Photo/FBI)

  • This frame grab from a video released by the FBI on Thursday, April 18, 2013, shows what the FBI are calling suspect number 1, black cap with backpack, walking in front of suspect number 2, white cap at lower right, through the crowd in Boston on Monday, April 15, 2013, before the explosions at the Boston Marathon. (AP Photo/FBI)

  • Suspect 2

  • This image from video released Thursday, April 18, 2013 by the FBI shows one of two suspects sought in connection with the Boston Marathon bombings. The FBI released photos and video of the two suspects and asked for the public's help in identifying them, zeroing in on the two men on surveillance-camera footage less than three days after the deadly attack. (AP Photo/FBI)

  • Suspect 2, Hand Next To Ear

  • This image from video released Thursday, April 18, 2013 by the FBI shows one of two suspects sought in connection with the Boston Marathon bombings. The FBI released photos and video of two suspects in the Boston Marathon bombing and asked for the public's help in identifying them, zeroing in on the two men on surveillance-camera footage less than three days after the deadly attack. (AP Photo/FBI)

  • Suspect 2

  • This image from video released Thursday, April 18, 2013 by the FBI shows one of two suspects sought in connection with the Boston Marathon bombings. The FBI released photos and video of two suspects in the Boston Marathon bombing and asked for the public's help in identifying them, zeroing in on the two men on surveillance-camera footage less than three days after the deadly attack. (AP Photo/FBI)

  • Boston Marathon Explosions

    This image provided by the Boston Regional Intelligence Center shows Dzhokhar A. Tsarnaev, one of the suspects in the Boston Marathon bombings. Authorities say Tsarnaev is still at large after he and another suspect — both identified to The Associated Press as coming from the Russian region near Chechnya — killed an MIT police officer, injured a transit officer in a firefight and threw explosive devices at police during their getaway attempt in a long night of violence into the early hours of Friday, April 19, 2013. The second suspect, who has not yet been identified, was killed in a shootout with police. (AP Photo/Boston Regional Intelligence Center)

  • This surveillance photo released via Twitter Friday, April 19, 2013 by the Boston Police Department shows a suspect entering a convenience store that police are pursuing in Watertown, Mass. Police say he is one of two suspects in the fatal shooting of an MIT police officer and tied to the Boston Marathon bombing. (AP Photo/Boston Police Department)

  • MIT SHOOTINGS 2

    Graphic shows photos of suspects; locates Watertown and Cambridge, Mass., where Boston Marathon bombing suspects exchanged gunfire and one is dead

  • This combo of photos released by the FBI early Friday April 19, 2013, shows what the FBI is calling suspects number 1, left, and suspect number 2, right, walking through the crowd in Boston on Monday, April 15, 2013, before the explosions at the Boston Marathon. (AP Photo/FBI)

  • This photo released by the FBI early Friday April 19, 2013, shows what the FBI is calling suspect number 1, walking through the crowd in Boston on Monday, April 15, 2013, before the explosions at the Boston Marathon. (AP Photo/FBI)

  • This photo released by the FBI early Friday April 19, 2013, shows what the FBI is calling suspect number 2, walking through the crowd in Boston on Monday, April 15, 2013, before the explosions at the Boston Marathon. (AP Photo/FBI)

  • This photo released by the FBI early Friday April 19, 2013, shows what the FBI is calling suspect number 2 behind a lady, walking through the crowd in Boston on Monday, April 15, 2013, before the explosions at the Boston Marathon. (AP Photo/FBI)

  • This photo released by the FBI early Friday April 19, 2013, shows what the FBI is calling the suspects together, walking through the crowd in Boston on Monday, April 15, 2013, before the explosions at the Boston Marathon. (AP Photo/FBI)

  • BOSTON MARATHON

    Graphic shows photos of the suspects and where they were caught on tape, a demographic profile of the runners and a map of the marathon path

  • Screen shot 2013-04-19 at 14

  • Tamerlan Tsarnaev practices boxing at the Wai Kru Mixed Martial Arts center in April 2009 in Boston, Mass. These images show what is believed to be Boston bomber Tamerlan Tsarnaev boxing at the Wai Kru Mixed Martial Arts center in Boston, MA. The native Chechnya, dubbed Suspect 1 by the FBI, and has been reported as dead. He was described as a heavyweight fighter at the gym, and allegedly hoped to fight for the U.S., because his home nation was not recognised in the sporting world.

  • Tamerlan Tsarnaev practices boxing at the Wai Kru Mixed Martial Arts center in April 2009 in Boston, Mass.

  • Tamerlan Tsarnaev practices boxing at the Wai Kru Mixed Martial Arts center in April 2009 in Boston.

  • Tamerlan Tsarnaev practices boxing at the Wai Kru Mixed Martial Arts center in April 2009 in Boston.

  • Tamerlan Tsarnaev

    Tamerlan Tsarnaev practices boxing at the Wai Kru Mixed Martial Arts center in April 2009 in Boston.

  • Tamerlan Tsarnaev practices boxing at the Wai Kru Mixed Martial Arts center in April 2009 in Boston.

  • Tamerlan Tsarnaev practices boxing at the Wai Kru Mixed Martial Arts center in April 2009 in Boston.

  • Tamerlan Tsarnaev