Two bombs exploded in the crowded streets near the finish line of the Boston Marathon on Monday, 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 U.S.

The explosions struck in Boston roughly three hours after the winning runners had crossed the finish line. The blasts hit the north side of Boylston Street, near the Fairmont Copley Plaza Hotel and the bridge that marks the finish line of the marathon course.

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.

In a press conference hours after the blasts Boston Police Commissioner Edward F. Davis said the death toll had risen from two to three.

“These were powerful devices that resulted in serious injury,” Davis told reporters. Massachusetts Governor Deval Patrick added that more than 100 people were injured, some gravely.

At the White House US President Barack Obama said it was not yet clear who was behind the blasts.