BOSTON - Police say at least three people have been killed in the explosions near the finish line of the Boston Marathon.
Police commissioner Ed Davis confirmed the three deaths but provided no details.
The explosions Monday also injured more than 130 people, knocking spectators and at least one runner off their feet.
Two bombs exploded in the packed streets near the finish line of the Boston Marathon on Monday leaving a bloody scene of shattered glass and severed limbs that raised alarms that terrorists might have struck again in the U.S.
A White House official speaking on condition of anonymity because the investigation was still unfolding said the attack was being treated as an act of terrorism.
President Barack Obama vowed that those responsible will "feel the full weight of justice."
A senior U.S. intelligence official said two other bombs were found near the end of the 26.2-mile course in what appeared to be a well-coordinated attack.
Authorities shed no light on a motive or who may have carried out the bombings, and police said they had no suspects in custody. Authorities in Washington said there was no immediate claim of responsibility.
"They just started bringing people in with no limbs," said runner Tim Davey of Richmond, Va. He said he and his wife, Lisa, tried to keep their children's eyes shielded from the gruesome scene inside a medical tent that had been set up to care for fatigued runners, but "they saw a lot."
"They just kept filling up with more and more casualties," Lisa Davey said. "Most everybody was conscious. They were very dazed."
The fiery twin blasts took place almost simultaneously 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 course.
When the second bomb went off, the spectators' cheers turned to screams. As sirens blared, emergency workers and National Guardsmen who had been assigned to the race for crowd control began climbing over and tearing down temporary fences to get to the blast site.
Blood stained the pavement, and huge shards were missing from window panes as high as three stories.
Boston police said two people were killed. Hospitals reported at least 134 injured, at least 15 of them critically. The victims' injuries included broken bones, shrapnel wounds and ruptured eardrums.
At Massachusetts General Hospital, said Alisdair 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."
Some 23,000 runners took part in the race, one of the world's oldest and most prestigious marathons.
One of Boston's biggest annual events, the race winds up near Copley Square, not far from the landmark Prudential Center and the Boston Public Library. It 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 had received "no specific intelligence that anything was going to happen" at the race.