Boston Marathon bombing

BOSTON >> Two bombs exploded in the crowded streets near the finish line of the Boston Marathon today, killing at least 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 United States.

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."

As many as two unexploded bombs were also found near the end of the 26.2-mile course as part of what appeared to be a well-coordinated attack, but they were safely disarmed, according to a senior U.S. intelligence official, who also spoke on condition of anonymity because of the continuing investigation.

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 route. 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 shield their children's eyes 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."

As the FBI took charge of the investigation, authorities shed no light on a motive or who may have carried out the bombings, and police said they had no suspects in custody. Officials in Washington said there was no immediate claim of responsibility.