BOSTON — The bombs that blew up seconds apart at the finish line of one of the world’s most storied races left the streets spattered with blood and glass, three dead, including an 8-year-old boy, more than 140 wounded and gaping questions of who chose to attack at the Boston Marathon and why.

Federal investigators said no one had claimed responsibility for the bombings on one of the city’s most famous civic holidays, Patriots Day. But the blasts among the throngs of spectators 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 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.”

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

A European security official said today 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 because he was not authorized to speak about the U.S. investigation.

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

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 today carrying brown paper bags, plastic trash bags and a duffel bag.

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

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.