Boston Marathon Bombings Update: FBI seeking suspects, motive in bombings that killed 3 and wounded more than 140 people

(CBS/AP) BOSTON - Federal investigators said no one claimed responsibility for the bombings at the Boston Marathon that left three dead, including an 8-year-old boy, and more than 140 wounded.

PICTURES: Explosions near Boston Marathon finish line

The FBI took charge of the investigations into the bombings, serving a warrant late Monday on a suburban Boston home and appealing for any video, audio and still images taken by marathon spectators.

President Barack Obama was careful not to use the words "terror" or "terrorism" as he spoke at the White House after the deadly bombings, but an administration official said the bombings were being treated as an act of terrorism.

"We will find out who did this. We'll find out why they did this," the president said. "Any responsible individuals, any responsible groups, will feel the full weight of justice."

A European security official said Tuesday initial evidence indicates that the attacks were not the work of suicide bombers.

"So far, investigators believe it was not the work of suicide bombers, but it is still too early to rule it out completely," said the official, who spoke from the United States on condition of anonymity.

The Pakistani Taliban, which has threatened attacks in the United States because of its support for the Pakistani government, denied any role Tuesday in the marathon bombings.

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 dense plumes of smoke rising over the street. Victims suffered broken bones, shrapnel wounds and ruptured eardrums.

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 25 to 30 people have at least one leg missing, or an ankle missing, or two legs missing."

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."

Massachusetts State Police confirmed that a search warrant related to the investigation into the explosions was served Monday night at an apartment in the Boston suburb of Revere, but provided no further details. Some investigators were seen leaving the Revere house early Tuesday carrying brown paper bags, plastic trash bags and a duffel bag.

Police said three people were killed. An 8-year-old boy was among the dead, according to a person who talked to a friend of the family and spoke on condition of anonymity. The person said the boy's mother and sister were also injured as they waited for his father to finish the race.

Hospitals reported at least 144 people injured, at least 17 of them critically. At least eight children were being treated at hospitals.

The Boston Marathon is one of the world's oldest and most prestigious races and about 23,000 runners participated. The race honored the victims of the Newtown, Conn., shooting with a special mile marker in Monday's race.

Boston Athletic Association president Joanne Flaminio previously said there was "special significance" to the fact that the race is 26.2 miles long and 26 people died at Sandy Hook Elementary School.

The marathon is one of the city's biggest annual events and is held on Patriots Day, which commemorates the first battles of the American Revolution, at Concord and Lexington in 1775.

Boston Police Commissioner Edward Davis asked people to stay indoors or go back to their hotel rooms and avoid crowds as bomb squads methodically checked parcels and bags left along the race route. He said investigators didn't know whether the bombs were hidden in mailboxes or trash cans.

He said authorities received "no specific intelligence that anything was going to happen" at the race.

The Federal Aviation Administration barred low-flying aircraft within 3.5 miles of the site.

"We still don't know who did this or why," Obama said at the White House, adding, "Make no mistake: We will get to the bottom of this."

With scant official information to guide them, members of Congress said there was little or no doubt it was an act of terrorism.

"We just don't know whether it's foreign or domestic," said Rep. Michael McCaul, R-Texas, chairman of the House Committee on Homeland Security.

Complete coverage of Boston Marathon explosions on Crimesider