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Two people were killed and more than 100 people were injured this afternoon as two powerful explosions detonated in quick succession near the Boston Marathon finish line in Boston’s Back Bay section, transforming a scene of athletic celebration into bloody chaos.

President Barack Obabma promised that whoever set off the explosives would be found and brought to justice.

“We still do not know who did this or why. And people shouldn’t jump to conclusions before we have all the facts,” he told reporters shortly after 6 p.m.. “But make no mistake. we will get to the bottom of this ... Any responsible individuals, any responsible groups will feel the full weight of justice.”

Blood and broken glass covered sidewalks in the area where the blasts occurred just before 3 p.m. Immediately after the explosions, some of the wounded could be seen to have lost limbs; others lay unconscious.

“It was just immediately [evident] there were injuries, right in the middle of the spectator crowds,” said boston.com sports producer Steve Silva, who was on the scene, covering the race. “There was blood everywhere; there were victims being carried out on stretchers. I saw someone lose their leg. People are crying. People are confused.”

Boston area hospitals reported that at least 115 people had been brought in for treatment from seven area hospitals. Brigham and Women’s Hospital received the most, with 26 brought in, including two in critical condition.

Asked if the explosions were a terrorist act, Boston Police Commissioner Edward F. Davis said, “We’re not being definitive on this right now, but you can reach your own conclusions based on what happened.’

The Federal Aviation Administration said it had implemented a no-fly zone around an area of Boston at the request of law enforcement officials and temporarily stopped planes on the ground at Logan International Airport to change the runway configuration.

Davis said the attacks had occurred without warning to officials.

Governor Deval Patrick said in a statement that it was a “horrific day in Boston” and that authorities were focusing on making sure the area around the Copley Square finish line was safe and secured. He urged everyone to stay away from the area.

Davis had said earlier that there had been a third explosion at the John F. Kennedy Library in Dorchester. But he downplayed those comments at a later news conference, suggesting that the incident might not have been an explosion.

Hundreds of police officers and firefighters descended on the scene immediately after the blasts, clearing the finish-line viewing stands. FBI, State Police, and agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives were among the investigators, officials said.

Bomb squads were searching the area, said Steve MacDonald, a spokesman for the Boston Fire Department. Officials said another explosion, heard in the city around 4 p.m., was a “controlled blast” staged as part of bomb squad activity.

“There were two bombs that exploded near the finish line in today’s Boston Marathon. We are working with law enforcement to understand what exactly has happened,” the Boston Athletic Association, the organization that runs the marathon, a magnet for runners from all over the world, said in a statement.

A Globe reporter on the scene saw people falling injured at two spots on Boylston Street, some of them knocked unconscious by the blast.

Davis, the police commissioner, said the two explosions had happened 50 to 100 yards apart at about 2:50 p.m.

MacDonald said that one explosion had happened in front of the Marathon Sports store at 671-673 Boylston and the windows were blown out at a LensCrafters store, where “the sidewalk is loaded with blood.”

Boston.com’s Silva said, “I was there at the finish, shooting finish line scenes, and then, bang!, it just went off, and then less than 15 to 20 seconds after, there was a second explosion, closer to Fairfield Street.”

“It was just an explosion. It came out of nowhere. ... I saw dismemberment, I saw blood everywhere. People are badly injured.”

Obama said he had directed the full resources of the federal government to “investigate and respond” to the blasts and that he had Patrick and Mayor Thomas M. Menino that “every single federal resource necessary” was available to them.

“The American people will say a prayer for Boston tonight,” Obama said. “We will find out wh did this and we will hold them accountable.

Mayor Thomas M. Menino said, “This is tragedy. We’re working together on this,” expressing his condolences to the families of those injured and killed.

Dave Benson, 41, was in the stands across Boylston Street from the explosion.

“I thought it was an end-of-the-event celebration with fireworks. Then I saw a huge plume of smoke and people falling down.”

Andrea George, 39, who was also across the street, said, “We heard a noise and heard the glass shatter. My friend was right there, and I can’t get in touch with them. I just started running. Everyone was running in different directions. It was the scariest thing I ever saw.”

Davis said that many people who fled the scene on Boylston left bags and parcels strewn along the route, and those items were being “treated as suspicious devices” and might be detonated by authorities.

Davis said he could not confirm reports that the bombs were placed in trash cans.

Davis also gave out two emergency phone numbers for people to call. Relatives of victims and anyone missing can call the mayor’s hotline at 617-635-4500, and anyone with information about the explosions should call police at 800-494-TIPS, he said.