BOSTON (AP) - The FBI's investigation into the bombings at the Boston Marathon was in full swing on Tuesday, with authorities serving a warrant on a suburban Boston home and appealing for any private video, audio and still images of the blasts that killed three and wounded more than 170.

Officials said no one had claimed responsibility for the bombings on one of the city's most famous civic holidays, Patriots Day. But the blasts that left the streets spattered with blood and glass raised fears of a terrorist attack.

President Barack Obama was careful not to use the words "terror" or "terrorism" as he spoke at the White House on Monday 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 on Tuesday that 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, on Tuesday denied any role in the marathon bombings.

The fiery explosions took place about 10 seconds and about 90 meters apart, knocking spectators and at least one runner off their feet, shattering windows and sending dense plumes of smoke rising over the street and through the fluttering national flags lining the route.

Blood stained the pavement, and huge shards were missing from window panes as high as three stories. Victims suffered broken bones, shrapnel wounds and ruptured eardrums.

Massachusetts Gov. Deval Patrick said on Tuesday that no unexploded bombs were found at the Boston Marathon. He said the only explosives were the ones that went off on Monday.

Roupen Bastajian, a state trooper from Smithfield, Rhode Island, 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."

WBZ-TV reported late Monday that law enforcement officers were searching an apartment in the Boston suburb of Revere.

Massachusetts State Police confirmed that a search warrant related to the investigation into the explosions was served Monday night in Revere, but provided no further details.

Some investigators were seen leaving the Revere house early on Tuesday carrying brown paper bags, plastic trash bags and a duffel bag.

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."

He said it remained to be determined what exactly the objects were.

Police said three people were killed. Eight-year-old Martin Richard 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.

Police commissioner Ed Davis said 176 victims were brought to hospitals around Boston, and 17 were in critical condition. At least eight children were being treated at hospitals.