UPDATED: 2 killed as 2 bombs explode at Boston Marathon, Third bomb goes off nearby

Two bombs exploded near the finish line of the Boston Marathon on Monday, killing two people and injuring at least 23 others in a terrifying scene of broken glass, smoke and severed limbs, authorities said.

A third blast rocked the John F. Kennedy Library several miles away and more than an hour later, but no injuries were reported, police said. A senior U.S. intelligence official said two other explosive devices were found near the marathon finish line.

There was no immediate word on the motive or who may have launched the attack and authorities in Washington said there was no immediate claim of responsibility. Some 27,000 runners took part in the 26.2-mile race, one of the world’s premier marathons.

The twin blasts at the race took place almost simultaneously and about 100 yards apart, tearing limbs off numerous people, knocking spectators and at least one runner off their feet, shattering windows and sending smoke rising over the street.

As people wailed in agony, bloody spectators were carried to a medical tent that had been set up to care for fatigued runners.

Cellphone service was shut down in the Boston area to prevent any possible remote detonations of explosives, a law enforcement official said. The official was speaking on condition of anonymity because the investigation was ongoing.

Boston Police Commissioner Edward Davis asked people to stay indoors or go back to their hotel rooms and avoid crowds as bomb squads checked parcels and bags left along the race route.

The Federal Aviation Administration barred low-flying aircraft from within 3.5 miles of the site.

“There are people who are really, really bloody,” said Laura McLean, a runner from Toronto, who was in the medical tent being treated for dehydration when she was pulled out to make room for victims of the explosions. “They were pulling them into the medical tent.”

About two hours after the winners crossed the line, there was a loud explosion on the north side of Boylston Street, just before the photo bridge that marks the finish line. Another explosion could be heard a few seconds later.

The Boston Marathon said that bombs caused the two explosions and that organizers were working with authorities to determine what happened. The Boston Police Department said two people were killed and 23 others injured. At least eight of the wounded were in critical condition, according to hospitals.

A senior U.S. intelligence official said the two other explosive devices found nearby were being dismantled. The official spoke on condition of anonymity because he was not authorized to discuss the findings publicly.