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Updated 5 p.m.: Boston Police Commissioner Ed Davis reported during a press conference that there are multiple causalities resulting from two explosions at the Boston Marathon, which occurred at 2:50 p.m. on Boylston Street. The explosions were 50 to 100 yards apart. Davis also reported that there was a third explosion at JFK library. He asking people in Boston to stay home and not to congregate in large crowds.

Two explosions went off near the finish line of the Boston Marathon on Monday as runners were finishing the race, injuring multiple people, witnesses told NBC News.

Video from the finish line showed screams and an enormous cloud of white smoke, and about 20 seconds between the blasts. The Associated Press reported that bloody spectators were being carried to a medical tent that had been set up to care for tired runners.

Jackie Bruno, a reporter for New England Cable News, said on Twitter that she saw people’s legs blown off.

“Runners were coming in and saw unspeakable horror,” she said.

NBC affiliate WHDH said that storefront windows nearby were blown out.

Will Ritter, the spokesman for a Massachusetts Senate candidate, told NBC News that he heard what sounded like two explosions and saw smoke rising near the Boston Public Library. He said that he saw three fire engines and police running to the site.

“We heard two really large explosions in rapid succession, about a second apart from each other,” Ritter said. “Everybody kind of ducked and hit the ground.”

Janet Wu, a reporter for WHDH, told NBC News that she heard two loud explosions. A witness told WHDH that it sounded like cannon fire.

Massachusetts General Hospital said that it had four patients and was expecting more.

The AP reported that runners and race organizers were crying as they fled the scene.

The Boston police bomb squad was en route. Boston police confirmed there was an incident but did not immediately give details. Police, fire and medical technicians were responding. New England Cable News reported that the emergency response came within seconds.

Federal authorities told NBC News that they had no immediate information. In New York, a spokesman told Reuters that police were deploying counterterrorism vehicles around landmarks, including prominent hotels.