A third fatality has yet to be identified. Officials say 176 people were injured when two bombs exploded near the finish line on Monday.
"We will go to the ends of the earth to identify the subject or subjects who are responsible for this despicable crime, and we will do everything we can to bring them to justice," Richard DesLauriers, FBI Boston, said.
In Boston, military hardware, soldiers, police and hundreds of federal agents took to the streets Tuesday. The 15-block crime scene remains littered with microscopic specks of potential evidence. Police may have found part of a circuit board that could help them determine how the bombs were made.
At a news conference, police and federal agents repeatedly appealed for any video, audio and photos taken by marathon spectators, even images that people might not think are significant.
"There has to be hundreds, if not thousands, of photos and videos" that might help investigators, state police Col. Timothy Alben said.
Boston Police Commissioner Edward Davis said investigators also gathered a large number of surveillance tapes from businesses in the area and intend to go through the video frame by frame.
"This is probably one of the most photographed areas in the country yesterday," he said.
Investigators refused to give any specifics on the bombs and say, for example, where they might have been hidden or whether they were packed with shrapnel for maximum carnage, as is often the case in terror bombings overseas.
Investigators say they have determined the area is safe, although it is still closed to the public. No additional bombs were found.
They also say a man who stopped by police after the explosion may be just a bystander who was trying to protect himself, like others. According to police, that man's apartment in suburban Revere was searched.
Death toll may climb
At least 17 people were critically injured, police said, and the death toll could climb. At least eight children were being treated at hospitals. In addition to losing limbs, victims suffered broken bones, shrapnel wounds and ruptured eardrums.
At Massachusetts General Hospital, Alasdair Conn, chief of emergency services, said: "This is something I've never seen in my 25 years here ... this amount of carnage in the civilian population. This is what we expect from war."
Eight-year-old Martin Richard was among the dead, said U.S. Rep. Stephen Lynch, a family friend. The boy's mother, Denise, and 6-year-old sister, Jane, were badly injured. His brother and father were also watching the race but were not hurt.
A candle burned on the stoop of the family's single-family home in the city's Dorchester section Tuesday, and the word "Peace" was written in chalk on the front walk.
Neighbor Betty Delorey said Martin loved to climb the neighborhood trees, and hop the fence outside his home.
Also killed was Krystle Campbell, a 29-year-old restaurant manager from Medford, Mass., who had gone with her best friend to take a picture of the friend's boyfriend crossing the finish line.
William Campbell said his daughter was "very caring, very loving person, and was Daddy's little girl."
Tim Davey of Richmond, Va., was with his wife, Lisa, and children near a medical tent that had been set up to care for fatigued runners when the injured began arriving. "They just started bringing people in with no limbs," he said.
"Most everybody was conscious," Lisa Davey said. "They were very dazed."
The Boston Marathon is one of the world's oldest and most prestigious races and about 23,000 runners participated. Most of them had crossed the finish line by the time the bombs exploded, but thousands more were still completing the course.
The attack may have been timed for maximum bloodshed: The four-hour mark is typically a crowded time near the finish line because of the slow-but-steady recreational runners completing the race and because of all the friends and relatives clustered around to cheer them on.
Davis, the police commissioner, said authorities had received "no specific intelligence that anything was going to happen" at the race. On Tuesday, he said that two security sweeps of the route had been conducted before the marathon.
The race winds up near Copley Square, not far from the landmark Prudential Center and the Boston Public Library. It is held on Patriots Day, which commemorates the first battles of the American Revolution, at Concord and Lexington in 1775.
Richard Barrett, the former U.N. coordinator for an al-Qaida and Taliban monitoring team who has also worked for British intelligence, said the relatively small size of the devices in Boston and the timing of the blasts suggest a domestic attack rather than an al-Qaida-inspired one.
"This happened on Patriots Day - it is also the day Americans are supposed to have their taxes in - and Boston is quite a symbolic city," said Barrett, now senior director at the Qatar International Academy for Security Studies.
The Pakistani Taliban, which has threatened attacks in the United States because of its support for the Pakistani government, on Tuesday denied any role in the bombings.
A woman who was a few feet from the second bomb, Brighid Wall, 35, of Duxbury, said that when it exploded, runners and spectators froze, unsure of what to do. Her husband threw their children to the ground, lay on top of them and another man lay on top of them and said, "Don't get up, don't get up."
After a minute or so without another explosion, Wall said, she and her family headed to a Starbucks and out the back door through an alley. Around them, the windows of the bars and restaurants were blown out.
She said she saw six to eight people bleeding profusely, including one man who was kneeling, dazed, with blood trickling down his head. Another person was on the ground covered in blood and not moving.
"My ears are zinging. Their ears are zinging," Wall said. "It was so forceful. It knocked us to the ground."
The Associated Press contributed to this report. All rights reserved.