The successive blasts went off less than a minute apart shortly before 3 p.m. near Copley Square, according to multiple reports.
SEE RELATED: Obama pledges support to Boston in wake of explosions
“There were two bombs that exploded near the finish line in today’s Boston Marathon,” the Boston Athletic Association said. “We are working with law enforcement to understand what exactly has happened.”
Exact numbers of casualties were not immediately clear, but Boston police reported two dead and more than two dozen injured. Various media outlets have the death toll higher.
“Our prayers are with those people in Boston who have suffered injuries,” Vice President Joseph R. Biden said. “I don’t know how many there are.”
Medical workers aid injured people at the finish line of the 2013 ... more >
Witnesses said they saw many people falling to the ground with bloody injuries from what one man described as a shrapnel-type bomb from the sidewalks.
“It didn’t come from the street, It didn’t come from a manhole,” the onlooker told NBC News.
The explosions were likely terrorist bombs and there could be more coming, an analyst warned.
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“The apparent Boston Marathon bombings are likely a terrorist attack,” said Ben Venzke of private sector intelligence outfit, IntelCenter.
Law enforcement sources told NBC that the first of two explosions appeared to be caused by a small homemade bomb, known as an IED (Improvised Explosive Device), perhaps hidden in a mailbox. The second explosion also makes it likelier that both were deliberate.
Mr. Venzke said he was closely monitoring extremist websites and chat rooms for any claim of responsibility or threat of further attacks.
“No credible claims have been identified so far,” he said, but if the perpetrators were a properly organized and experienced terrorist group, “there is the distinct possibility of additional coordinated attacks in Boston and/or other major cities.”
Viewers who tuned into daytime TV programming such as the Ellen DeGeneres Show and Dr. Oz were instead greeted by urgent special reports on what had happened at the popular race.
TV footage showed emergency workers ripping away barrier fencing near the site of the blast, where white smoke billowed and national flags lined up to adorn the occasion also fell to the ground. Runners and spectators alike ran in criss-crossing directions to and from the scene.
Competitors and race organizers were crying as they fled the chaos. Bloody spectators were being carried to the medical tent that had been set up to care for fatigued runners.
Explosions rock finish of Boston Marathon; 2 killed and at least 23 hurt, police say
Two explosions described by race organizers as bombs went off near the finish line of the Boston Marathon on Monday as runners completed the course and thousands cheered them on. At least two people were killed and 23 injured, police said.
Video from the finish line showed screams and an enormous cloud of white smoke, and about 20 seconds between the blasts. The explosions were strong enough to cause at least one runner to collapse, and emergency personnel carried bloody spectators away. A witness said that she saw people’s legs blown off.
“We saw two big puffs. I thought maybe it was fireworks. Then it went off again. And then all of a sudden we heard people crying and running away. It was a huge horde of people just running away,” said Serghino Rene, another witness, who was a few blocks away. “We just got away from the scene and away from tall buildings.”
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A third, undetonated device was found near the finish line, a House Homeland Security Committee official and three law enforcement officials told NBC News.
The injury count from area hospitals exceeded that initially given by the police. Massachusetts General Hospital said that it had 19 patients, and Tufts Medical Center said it had nine. Jackie Bruno, a reporter for New England Cable News, said on Twitter that she saw people’s legs blown off.
“Runners were coming in and saw unspeakable horror,” she said.
Race organizers, in a post on Twitter, said: “There were two bombs that exploded near the finish line in today’s Boston Marathon. We are working with law enforcement to determine exactly what happened.”
President Barack Obama was notified, and his administration was in touch with state and local authorities, a White House official told NBC News. Obama directed the administration to provide whatever help was necessary, the official said.
Within an hour of the blasts, New York police deployed extra security to landmarks, and Pennsylvania Avenue in front of the White House was closed to foot traffic. San Francisco put its police on heightened alert.
Related: Images from the scene
The race is a signature event in Boston and has been run since 1897 on Patriots Day, the third Monday in April. Tens of thousands of spectators turn out each year to watch.
Race organizers said that almost 27,000 runners competed, representing 96 countries. The winners were Lelisa Desisa of Ethiopia for the men and Rita Jeptoo of Kenya for the women. A special marker at the 26th mile of the course, yards from the finish, had been set up to honor the 26 dead in the Newtown, Conn., elementary school shooting last December.
The race began at 10 a.m., and the explosions were reported just before 3 p.m. The winners had long ago completed the race — Desisa finished with a time of just over 2 hours, 10 minutes — but the explosions came as masses of other runners were approaching the finish. NBC affiliate WHDH said that storefront windows nearby were blown out.
Related: Slideshow of scene of explosions
“Right now I’m in my condo with about 50-60 people I picked up off the street including marathon runners. Setting up a camp,” Corey Griffin told NBC News. “They have nowhere to go because everything is shut down. Officials said to get inside. This is crazy.”
Will Ritter, the spokesman for Massachusetts Senate candidate Gabriel Gomez, told NBC News that he heard what sounded like two explosions and saw smoke rising near the Boston Public Library. He said that he saw three fire engines and police running to the site.
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“We heard two really large explosions in rapid succession, about a second apart from each other,” Ritter said. “Everybody kind of ducked and hit the ground.”
Another witness told WHDH that it sounded like cannon fire.
The AP reported that runners and race organizers were crying as they fled the scene, and that bloody spectators were carried to medical tents intended for exhausted runners. Runners who were still on the 26.2-mile course were being stopped and directed elsewhere, the Massachusetts Emergency Management Agency said. The agency suggested that people trying to reach loved ones use text messaging because of crowded phone lines.
New England Cable News reported that the emergency response came within seconds.
The Boston transit system suspended some service.
Tracy Connor of NBC News contributed to this report. Reuters also contributed to this report.