One of the dead was an 8-year-old boy, a law enforcement source confirmed.
LATEST FACTS:
- Three dead, including an 8-year-old boy
- At least 140 injured, with at least 15 in critical condition
- Explosions were almost simultaneous, caused by small portable devices
- Dozens of law enforcement officers descended on Revere, Mass., building
- Incident at JFK Library was still under investigation
Copley Square was jammed with tens of thousands of spectators when the two explosions went off in quick succession. The first explosion was heard on the north side of Boylston Street at Marathon Sports running store at 2:50 p.m. ET. It was unclear if the explosion was inside or outside, but it blew out windows in four buildings, officials said. Then about 10 seconds later, another explosion could be heard about 50 to 100 yards away, severely injuring more bystanders.
An official told ABC News that the explosions were caused by small portable devices. As many as two unexploded bombs were also found near the end of the 26.2-mile course as part of what appeared to be a well-coordinated attack, but they were safely disarmed, according to a senior U.S. intelligence official, who also spoke on condition of anonymity because of the continuing investigation.
Sources confirmed that investigators were interviewing a man at a Boston hospital, but it was too early to determine whether he was simply an important witness or a person of interest.
Law enforcement officials were interviewing a person of interest in the case, ABC News learned. The 20-year-old Saudi man in Boston on a student visa was not characterized as a suspect, according to Boston police. The man has no apparent criminal history, according to sources. Sources tell ABC News authorities are looking for any friends and associates of the 20-year-old Saudi.
Police were questioning many people, according to sources.
Dozens of law enforcement officers searched an apparently vacant fifth-floor apartment in the Boston suburb of Revere, according to ABC News. Eyewitnesses said they were told the search was related to the bombings.
It was not yet clear who was behind the attack. President Barack Obama was immediately notified about the explosions and has directed his administration to provide whatever assistance is necessary in the investigation and response.
"We will find out who did this. We'll find out why they did this," Mr. Obama said. "Any responsible individuals, any responsible groups, will feel the full weight of justice."
There was a fire a few miles away from the finish line at John F. Kennedy Library. Boston Police Commissioner Edward Davis said that it may have been caused by an incendiary device, but that it was not clear whether it was related to the bombings. The extent of the damage there was not clear, and police say there were no injuries that they know of at JFK Library.
Davis urged people to stay home and avoid crowded areas while authorities conduct the investigation. The National Guard helped secure the perimeter and the FBI took charge of the investigation.
When the marathon explosions went off, thousands of people fled from the area, and many left bags and personal items at the scene. Davis said each of the items was being treated as a suspicious device.
The L.A. Running Club sent 10 members to Boston to run the marathon. According to the club's vice president, Alan Culver, all the runners finished in 3.5 hours or sooner, so they were all expected to be OK. The two blasts happened right after the four-hour mark.
All victims were removed from the scene. At least 15 of the injured were in critical condition, according to hospital officials. The wounded were shuttled out of the area on wheelchairs, some of them with blood on their faces.
"I started running toward the blast. And there were people all over the floor," said Roupen Bastajian, a 35-year-old state trooper from Smithfield, R.I., who had just finished the race when he heard the first blast. "We started grabbing tourniquets and started tying legs. A lot of people amputated. ... At least 25 to 30 people have at least one leg missing, or an ankle missing, or two legs missing."
A plume of smoke was seen after the explosions and had a sulfur smell, witnesses said. Cellphone service was shut down in the Boston area to prevent any possible remote detonations of explosives.
The Federal Aviation Administration has also barred low-flying aircraft from within 3.5 miles of the site.
"There are a lot of people down," said one man, whose bib number identified him as Frank Deruyter of North Carolina.
Shortly after the blasts, police in major cities worldwide increased their security. In Los Angeles, there are increased patrols for transit and other critical areas like the Dodger Stadium, according to sheriff's spokesman Steve Whitmore.
There were more than 24,000 participants who streamed across the starting line in Hopkinton on Monday morning for the 117th edition of the Boston Marathon.
The Boston Marathon is held on Patriots Day, which commemorates the first battles of the American Revolution, at Concord and Lexington in 1775.
ABC News and The Associated Press contributed to this report.
