"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.
The fiery explosions took place about 10 seconds and about 100 yards apart, knocking spectators and at least one runner off their feet, shattering windows and sending columns of smoke rising over the street.
Roupen Bastajian, a state trooper from Smithfield, R.I., had just finished the race when he heard the explosions.
"I started running toward the blast. And there were people all over the floor," he said. "We started grabbing tourniquets and started tying legs. A lot of people amputated."
At least 17 people were critically injured, police said. At least eight children were being treated at hospitals. In addition to losing limbs, victims suffered broken bones, shrapnel wounds and ruptured eardrums.
Dr. Stephen Epstein of the emergency medicine department at Beth Israel Deaconess Medical Center said he saw an X-ray of one victim's leg that had "what appears to be small, uniform, round objects throughout it — similar in the appearance to BBs."
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.