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.
"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.
PHOTOS: Boston Marathon Explosions
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.
LIVE BLOG: Explosions at Boston Marathon
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.