BOSTON (AP) - Two bombs exploded in the packed streets near the finish line of the Boston Marathon on Monday, killing two people and injuring more than 100 in a terrifying scene of shattered glass, bloodstained pavement and severed limbs, authorities said.
A senior U.S. intelligence official said two other bombs were found near the end of the 26.2-mile course.
President Barack Obama vowed that those responsible will "feel the full weight of justice."
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.
Authorities shed no light on a motive or who may have carried out the attack, 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 assigned to the race for crowd control began climbing over and tearing down temporary fences to get to the blast site.
A pool of blood formed, and huge shards were missing from window panes as high as three stories.
Boston police said two people were killed. Hospitals reported at least 105 injured, at least 15 of them critically.
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.