BOSTON (USA TODAY) -- Two bomb blasts ripped through the Boston Marathon's crowded finish line Monday afternoon, killing at least three people and injuring more than 141.
The dead included an 8-year-old boy, The Boston Globe reported, citing two law enforcement sources briefed on the investigation. Among the injured, 17 were reported in critical condition at eight local hospitals.
"There were so many people in that area that they couldn't get ambulances in there," said Joe Difazio, who was working on communications near the site when the blasts occurred. "They were wheeling people out in wheelchairs. One guy had no legs. The bones was just sticking out... It was horrible."
PHOTO Gallery: Several Injured following explosion at Boston Marathon
WEB EXTRA: USA Today's coverage on the Boston Marathon Explosions
Witnesses described a blast scene of widespread mayhem, with victims suffering shrapnel wounds that severed several victims' limbs.
The FBI is overseeing the investigation, which Special Agent Rick Deslauriers said involved possible terrorism. He would not comment on specific suspects or leads in the case. "It is a very active, fluid investigation at this time,'' he said.
Speaking from the White House, a somber President Obama said people should not speculate over who was responsible.
"We still don't know who did this or why. People should not jump to conclusions before we have all the facts. But make no mistake. We will get to the bottom of this. We will find out who did this. We will find out why they did this. Any individual or responsible groups will feel the full weight of justice."
The two bombs exploded about 12 seconds and 100 yards apart. Police also detonated a suspicious package found along the race course, but it did not appear to have been an explosive device, Boston Police Commissioner Ed Davis said at a news briefing.
A federal law enforcement official said investigators are examining about three other possible devices, including one recovered in the Newton area. The official, who has been briefed on the matter but is not authorized to comment publicly, said investigators were still attempting to determine whether the material represented a threat.
The devices were described by the official as relatively small and possibly containing small ball bearings or BB gun pellets designed to serve as shrapnel. It was unclear whether the devices were remotely detonated or included timers, the official said, adding that no conclusions had been drawn on whether an organized group or lone wolf had been responsible for the attack.
The official said authorities were questioning one person who was seen fleeing from the scene and sought treatment at a local hospital. But it was not known whether the person represented more than a witness to the incident.
Multiple media reports indicated a person of interest was being held at an undisclosed hospital. However, Davis said investigators are talking to several people but "there is no suspect at this time."
A third incident at the John F. Kennedy Presidential Library was initially described as a third explosion, but Davis said Monday night that it may have been only a fire. No injuries were reported, but nearby universities were being evacuated.
Davis said the first two explosions occurred in quick succession, 50 to 100 yards apart, about 2:50 p.m., three hours after the winner had crossed the finish line.
"We're encouraging people to not go out, if they're in hotels to stay in their rooms," Davis said.
"After this incident occurred, there were a lot of people running from the scene, a lot of them deposited bags and parcels," Davis said. "Each one is being treated as a suspicious device. At this point, we haven't found any more devices."
Bloodied spectators were carried to a medical tent intended for runners. At least one police officer was hurt.
Organizers stopped the race and locked down the marathon headquarters.
The Federal Aviation Administration announced a temporary flight restriction over Boston.
Cellphone service was operating, wireless companies reported, contrary to an earlier Associated Press quoting a law enforcement official who said service was cut in case there were other undetected devices.
The elite women runners started the race at 9:30 a.m. and the elite men followed about 30 minutes later. About 27,000 runners were in the field for the Patriots Day race.
Nancy Costa, a medical student from Reading, Pa., was running with her friend Jill Edmonds of Salem, N.H., when the explosions erupted.
"It was insane here. Everyone was running. I was right next to the explosion. It threw me," she said. "I never sprinted so fast after a marathon.
"The first (blast) threw me onto the ground. And everything went silent and then the second went off and I just covered my head and got up and started sprinting. Everyone was screaming and people were getting trampled. We finally found an open T (subway train) that just arrived in Wellington (station). We had to walk a few miles to find one open."
Kimberly DelGuzzi of Pittsburgh was waiting on Boylston Street for her friend to cross the finish line when she found herself pressed against a building, ducking for cover from the blasts.
"At first, I thought it was fireworks, but then I saw the smoke go up in the air," said DelGuzzi, who was standing between the two explosions. "Then, not even a minute later, the second one went off."
She described the scene as "mass chaos" and said, "Oh my God, it was loud."
"The explosions shook everything," she said, her voice still shaking 40 minutes after the bombs went off. "I saw runners down in the street. I saw people down on the sidewalk."
DelGuzzi, 41, has run numerous marathons but was not running in Boston. Her friend reported she was OK.
Jim Davis, one of the marathon's official photographers, told the Des Moines Register he was about 50 feet from where the first blast ripped through a glass storefront.
"Debris was falling. Fortunately I was far enough away that there weren't any glass shards," said Davis, 65, of Fairfield, Iowa. "Then people started running and screaming and I realized this is not an accident - I should get out of there."
After the second explosion, about a block away, Davis returned for his camera gear and saw one man who had lost both his legs and others were severely cut.
"I'm not a war correspondent," he said. "I'm not used to seeing people blown up with injuries."
Tom Beusse, president of the USA TODAY Sports Media Group, had just finished the race and was about 150 yards away from the explosion.
"I finished the race and began to walk through the corrals," he said. "All of a sudden there was this giant explosion. All of us turned around, the runners, and had these looks on their faces like 'Oh my god.' ... Immediately, it turned into mayhem. People were screaming. Cops told us to keep moving away from the finish line in the direction we were going. No one knew what was coming next -- and thankfully, nothing was next."
Massachusetts General Hospital was treating 19 victims, spokeswoman Susan McGreevey said. Six were in surgery in critical condition, four suffering "traumatic amputations" from having legs cut off by the force of the explosions.
Tufts New England Medical Center had nine patients "and we're expecting more," said spokeswoman Julie Jette. Brigham and Women's Hospital reported receiving 18 to 20 injured from the explosions, two in critical condition.
Police evacuated a 15 block area around Copley Square and bolstered security around the city.
Security was also increased around Washington and New York.
The New York Police Department has stepped up security around landmarks in Manhattan, including near prominent hotels, said Paul Browne, deputy commissioner of the NYPD.
Browne told Reuters that New York police were re-deploying counter-terrorism vehicles around the city.