Two blasts near the finish line of the Boston Marathon killed two people and injured dozens on Monday, though the scope of the blasts is still unclear.

Witnesses reported hearing a loud explosion near the finish line just before 3 p.m. 

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. Boston officials said about 90 people were injured. 

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. Police in Los Angeles, New York City, London and other cities worldwide stepped up security Monday following explosions at the Boston Marathon.

"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."

A senior U.S. intelligence official said two additional explosive devices were been found near the scene of the Boston marathon and were being dismantled.

 Roupen Bastajian, a 35-year-old state trooper from Greenville, R.I., had just finished the race when they put the heat blanket wrap on him and he heard the first blast.

"I started running toward the blast. And there were people all over the floor," he said. "We started grabbing tourniquets and started tying legs. A lot of people amputated. ... At least 25 to 30 people have at least one leg missing, or an ankle missing, or two legs missing."

A Boston police officer was wheeled from the course with a leg injury that was bleeding.

Smoke rose from the blasts, fluttering through the national flags lining the route of the world's oldest and most prestigious marathon. TV helicopter footage showed blood staining the pavement in the popular shopping and tourist area known as the Back Bay.

"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."

Cherie Falgoust was waiting for her husband, who was running the race.

"I was expecting my husband any minute," she said. "I don't know what this building is ... it just blew. Just a big bomb, a loud boom, and then glass everywhere. Something hit my head. I don't know what it was. I just ducked."

Runners who had not finished the race were diverted straight down Commonwealth Avenue and into a family meeting area, according to an emergency plan that had been in place.

Based on reporting by The Associated Press.

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