BOSTON - Investigators appealed to the public Tuesday for amateur video and photos that might yield clues to the Boston Marathon bombing as the chief FBI agent in Boston vowed "we will go to the ends of the Earth" to find whoever carried out the deadly attack.
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.
A doctor treating the wounded said one of the victims was maimed by what looked like ball bearings or BBs.
Federal investigators said no one had claimed responsibility for the bombings, which took place on one of the city's biggest civic holidays, Patriots Day. But the blasts raised the specter of another terrorist attack on U.S. soil.
President Barack Obama said the bombings were an act of terrorism, but investigators do not know if they were carried out by an international organization, domestic group or a "malevolent individual." He added: "The American people refuse to be terrorized."
Across the U.S., from Washington to Los Angeles, police tightened security, monitoring landmarks, government buildings, transit hubs and sporting events. Security was especially tight in Boston, with bomb-sniffing dogs checking Amtrak passengers' luggage at South Station and transit police patrolling with rifles.
"They can give me a cavity search right now and I'd be perfectly happy," said Daniel Wood, a video producer from New York City who was waiting for a train.
Richard DesLauriers, FBI agent in charge in Boston, said investigators had received "voluminous tips" and were interviewing witnesses and analyzing the crime scene.
"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," he said.
Gov. Deval Patrick said that contrary to earlier reports, no unexploded bombs were found. He said the only explosives were the ones that went off.
FBI agents searched an apartment in the Boston suburb of Revere overnight, and investigators were seen leaving with brown paper bags, plastic trash bags and a duffel bag. But it was unclear whether the tenant had anything to do with the attack.
A law enforcement official who spoke on condition of anonymity because he was not authorized to release details of the investigation said the man had been tackled by a bystander, then police, as he ran from the scene of the explosions.
But he said it is possible the man was simply running away to protect himself from the blast, as many others did.
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.
But 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."
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.
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