BOSTON (AP) - Two bombs exploded in the crowded streets near the finish line of the Boston Marathon on Monday, killing three people and injuring more than 140 in 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.

The fiery twin blasts 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 course. Blood stained the pavement, and huge shards were missing from window panes as high as three stories.

"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."

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. The FBI took charge of the investigation.

Police said three 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, 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.

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.

A few miles away from the finish line and around the same time, a fire broke out at the John F. Kennedy Library. The police commissioner said that it may have been caused by an incendiary device and that it was not clear whether it was related to the bombings.

The first explosion occurred on the north side of Boylston Street, just before the finish line.

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.

The bombings occurred about four hours into the race and two hours after the men's winner crossed the line.

By that point, more than 17,000 of the runners had finished the race but thousands more were still running.

The attack may have been timed for maximum carnage: The four-hour mark is typically a crowded time near the finish line because of the slow-but-steady recreational runners completing the race and because of all the relatives and friends clustered around to cheer them on.