The FBI are examining images of a grey and orange bag pictured on the Boston Marathon route where the second bomb was detonated.
The images, taken by a member of the public, were submitted to NBC station WHDH and have been passed to the authorities for review.
The before shot shows the bag next to a mailbox along a barricade, NBC US News reports.
The after image, taken around an hour later, seems to show no sign of the bag. The surrounding area has been blurred due to its graphic nature.
Meanwhile the first images of one of the bombs has been obtained by the Associated Press.
It shows 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.
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.
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.
President Barack Obama confirmed the FBI was investigating the tragedy as an act of terrorism, as it emerged that some 170 people were injured in the explosions.
Mr Obama said: "We will find out who did this. We'll find out why they did this. Any responsible individuals, any responsible groups, will feel the full weight of justice.
"This was a heinous and cowardly act. The FBI is now investigating this as an act of terrorism."
But he added: "The American people refused to be terrorised. What the world saw yesterday in the aftermath of the explosions were stories of heroism and kindness and generosity and love."
Richard DesLauriers, the special agent in charge of the Boston FBI office, told a press conference: "This will be a worldwide investigation. We will go to the ends of the earth to find the subject or subjects responsible for this despicable crime.
"Our mission is clear - to bring to justice those responsible for the marathon bombing," he added.
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"This remains a very active investigation, our ongoing investigation in various locations throughout the area goes on.
"However, there are no known additional threats. We continue to interview various witnesses and process the crime scene, which could take some time."
He said the FBI had received "voluminous" tip-offs over the last 18 hours.
Scores of victims were hurt, many suffering shrapnel wounds, in the two explosions, which were caused by bombs believed to be stashed in bins nearby.
Massachusetts General Hospital said "several amputations" had to be performed on survivors.
Horrifying images of injured spectators and blood-spattered pavements were caught as television cameras broadcast footage of the explosions.