BOSTON (ABC) - The death toll in the Boston Marathon bombings has increased to three people, according to the Boston Police commissioner.
Among the dead was an 8-year-old boy, law enforcement sources told ABC News. The Boston Globe identifies the dead child as Martin Richard.
"This cowardly act will not be taken within stride," Boston Police Commissioner Ed Davis said this evening. "We will turn every rock over to find the people who are responsible for this."
At least 133 people were injured, including several children with severe trauma, when bombs exploded almost simultaneously near the marathon finish line, police said.
Doctors at two Massachusetts hospitals said some of the victims underwent amputations and were suffering from burns, while others had sustained injuries from shrapnel to their lower extremities.
"Everything we saw was ordinary material that could have been propelled by the device," said Dr. Ron Walls, chair of emergency medicine at Brigham and Women's Hospital.
FBI Taking Charge of Investigation
The FBI has taken the lead in the investigation and at a news conference tonight said there are no suspects in custody.
Despite reports of law enforcement officials questioning a potential person of interest at Brigham and Women's Hospital, where many of the injured were taken, Davis urged caution.
"There's no suspect at Brigham and Women's Hospital," Davis said. "There are people we are talking to, but no suspect."
ABC News can confirm that one of the people law enforcement officials are talking to is a 20-year-old Saudi national at a Boston hospital. Sources tell ABC News that he is here legally on a student visa and that his visa is clean with no apparent criminal history.
Boston police tonight, at a news conference, said that there are people they are talking to, but no suspects.
Two bombs exploded near the race finish line on Boylston Street shortly before 3 p.m. The area was crowded with runners and spectators, and thousands of runners were still completing the race.
Davis declined to say whether he thought the devices were acts of terrorism, but said, "You can reach your own conclusion based on what happened."
According to law enforcement sources, the first bomb exploded at the Marathon Sports running store and blew out windows in four nearby buildings, injuring 15 to 20 individuals. The second blast occurred about 50 to 100 yards away, severely injuring more bystanders, Davis said.
The working theory about the bombs among Massachusetts law enforcement is that they were small, crudely made devices hidden in bags or backpacks, planted either during the race or immediately prior and detonated remotely, possibly with a cellphone. Officials have subpoenaed cellphone records.
More than 400 National Guardsmen in attendance at the marathon helped secure a perimeter around the scene.
One witness described the scene as being like a "war zone," while a doctor who was standing nearby said he immediately started treating people with severe leg injuries.
"Six or so people went down right away on my left, mostly with leg injuries. One gentleman had both legs below the knee blown off," Dr. Allan Panter, a physician who witnessed the event, told ABC News. "One girl I treated, I could not find any obvious injury to her torso, but she arrested. She was between 24 and 30.
"The people had singed facial hair and stuff. Most of the injuries were on their legs," Panter said. "I was 20 feet away, one storefront down. My ears were ringing. Everything blew out from the storefront."
Police initially said a third explosion occurred at John F. Kennedy Library in Boston, but later said it was related to a fire. No one was injured at the library, police said.
The Federal Aviation Administration issued a temporary flight restriction over the area of the explosion.
Police were asking for all video footage of the finish line at the time of the explosion.
An emergency room doctor at Massachusetts General Hospital told ABC News that the hospital had performed several amputations, particularly on victims whose legs were injured. Many of the victims were runners still wearing numbers on their shirts, the doctor said.
He described the injuries as "shrapnel-type wounds" possibly caused by "pipe bombs," though police have not confirmed that description.
Earlier, a trauma nurse from Massachusetts General Hospital told ABC News that medical workers had set up a temporary morgue at a medical tent at the road race and were treating patients with severed limbs and children with severe burns.
In the immediate aftermath of the explosions, Boston EMS personnel could be seen shuttling the injured out of the blast area on wheelchairs. Several of the victims were bleeding from the face.
A doctor who was in the medical tent about 150 yards away from the explosion said it looked like a "war zone," with "lots of blood," and said that all physicians were told to go to the scene and help the injured.
Boston police set off a third explosion before 4 p.m. and were sweeping the area, checking dozens of bags left behind by runners who evacuated the area after the explosions. Officials also tested for chemicals to help determine what kind of device was used, according to police.
Attorney General Eric Holder was in touch with the FBI in Boston and President Obama was notified of the blasts. All of Boston's police force was ordered to report to duty.
Security precautions were taken elsewhere beyond Boston. In Washington, D.C., Pennsylvania Avenue in front of the White House was closed to pedestrians and there was heightened security.
In Boston, police told people in area of the blasts to avoid trash cans, according to witnesses.
The explosions erupted on what is usually a festive day in Boston. It is designated Patriots Day and most offices are closed for the celebration and the marathon.
Debris from the explosions could be seen scattered throughout the spectators' stands and finish line area of the marathon as emergency personnel cleared the area.
Video of the explosions showed plumes of white smoke pouring into the air above the street where runners were.
More than 26,00 runners were registered to compete in this year's marathon. The marathon clock was at shortly after four hours at the time of the explosions, which is the average time it takes runners to complete the Boston race, potentially putting the greatest number of competitors at risk.
ABC News - BOSTON
Eyewitnesses at the Boston Marathon described the ground shaking beneath their feet, smoke and "a lot of chaos" this afternoon when explosions killed two people and injured dozens.
Rachel Sibley, 22, was 50 feet from the finish line waiting for a friend to cross.
"All of a sudden I heard this bang that sounded like a cannon," she said. "You could see people looking up at the sky like there were fireworks, like a celebratory bang. The whole crowd waited for a moment, and then the second one went off. It was terrifying and absolute chaos.
"Everyone needed to get out of there," said Sibley. "I was ready to just start running, otherwise I'd be tackled. You could see the panic in people's faces.
"People started screaming and yelling at each other, trying to find friends and family members," she said. "People started running away from the finish line. There were sirens filling the streets and heading back to the finish line. It was absolutely terrifying."
One doctor in a medical tent about 150 yards from the explosion said he was immediately mobilized.
"We all went running over there and started to bring people into the medical tent," he said. "It was not good. Very bad. Like a war zone. 9/11 immediately came to mind."
He said the subways were immediately shut down and people were walking out of the area.
One eyewitness, Joe Conway, said he saw a giant cloud of gray smoke and ran to the food court at the Prudential Center.
"People were running out and didn't know what was going on," he told ABC News. "I had a baseball hat on and could feel the concussion. The baseball hat fell off my head."
Amanda Fahkkredine, 25, had just walked down into the Arlington subway station a few blocks from the finish line when the bombs went off. As two trains were pulling into the station, she heard it.
"You heard this huge noise and a rumble, and then two T transit police started yelling at everyone to get out of the station," Fahkkredine said. "They didn't seem to know what was going on."
Marathoners began hobbling up the subway stairs. Fahkkredine asked transit police what happened, but they told her they didn't know -- that everyone just had to get out.
"We heard it, and we no idea if there was a train crash or car accident," she said. "It wasn't like anything that I've ever heard or felt before. It was like an earthquake sounding like a car crash."
Back above ground, officers told pedestrians to stay away from landmarks.
Fahkkredine finally got word from her boyfriend that it was a bomb. Although Fahkkredine is not a Boston native, she graduated from Boston University in 2010 and calls the city home.
"I can't believe that just happened," she said. "It's shocking."
The first blast was reported at 2:42 p.m. near the finish line medical tent. By 3:15, authorities said a second device was reported at Saint James Avenue and Trinity Place.
The race was stopped about 3:28 p.m. EMS was tagging everyone affected with red wristbands and beginning transports to Boston hospitals.
A controlled explosion occurred at 4 p.m.
"People were running and screaming and crying in the area," college student Dan Lamariello told ABC News. "There was trampling, running It was a very scary scene."
Racer Jill Elaine Czarnik, 24, of Chicago, finished just 20 minutes after the explosion went off. She was standing in the hospitality area about a quarter mile from the finish line.
"I was in the first wave and I think most people [when the explosions went off] were in the second wave," said Czarnik, who didn't hear the explosion. "It's kind of like a movie scene. ... I think everyone is just kind of like in shock. ... I still feel a little shaky, but I feel safe now. ... It's just very weird ... because you don't know what it is and you're just very delusional because you just ran a marathon."
Phone lines were too busy for many people in the area to reach friends and relatives.
They're saying just send texts and not to make phone calls so people can reach their loved ones," said Czarnik.
Dan Lau of Boston, a 25-year-old engineer running his sixth marathon, said he was three or four blocks away from the chaos, crossing the finish line about 20 minutes earlier.
"It sounded like something from out of a movie," he said.
"Initially, folks were running towards a narrow, fenced-in exit spilling over to Newbury, but several bystanders diffused the panic and calmed us down," he said. "My first impression was either a truck hit a building or a subway train crashed. Then, when the second went off, my friend and I knew it was more serious. My friend immediately thought it was a bomb. To quote him, he said, 'That's definitely not thunder.'"
Runners further back were stopped by authorities. Josh Crary, one of 40 blind or visually impaired runners, had to stop at mile 20 in suburban Newton, west of the city.
"We stopped after my sister gave me a phone call," said Crary, a 27-year-old from Barnstead, N.H. "She said there had been an explosion and people were running away. Ten minutes later, the race was shut down."
His sister was in a VIP booth at the finish line. Crary said he and his guide were taken in by neighbors.
"Some folks took us into their home," he said. "It's almost impossible to call people because cell phones are busy. Hopefully, this is some sort of freak accident or explosion and not some other terrible scenario."
A Google document circulated on which Bostonians listed their contact information and offered up places for marathon runners to stay. As of three hours after the explosions, 708 people had offered up their homes.
As for not being able to finish the race, Crary said, "I could care less about the finish. There are much bigger things to worry about."
ABC News' Lauren Effron, Liz Neporent, Susanna Kim and Kelley Robinson contributed to this report.