Two people were killed and at least 100 injured after two explosions went off near the finish line of the Boston Marathon on Monday as runners completed the race and thousands of people cheered them on.
Video from the scene showed two blasts about 20 seconds apart just off the course at the finish. The explosions occurred in the area of Boylston Street, police confirmed, as thousands of runners finished the 117th running of the Boston Marathon, with crowds watching and cheering at the finish. A third undetonated device was found near the finish line, a House Homeland Security Committee official and three law-enforcement officials told NBC News.
"All the sudden there was a massive boom. There was a sort of concussive blow that pushed a lot of people back. I could see runners falling in front of me," said Dave Abel, a reporter for the Boston Globe who was 10 feet from one of the explosions.
"When the smoke started to clear, I could see lots of bodies," he said. "I could see one woman staring vacantly into the sky. I could see a lot of mangled limbs, a lot of blood and shattered glass. It was probably the most horrific thing I've ever seen."
Larissa Brinkley, who came from Pennsylvania to run the race, said people dropped everything and ran the opposite way. Other witnesses described what at first sounded like a cannon blast or fireworks.
"Then it went off again. And then all of a sudden we heard people crying and running away," said Serghino Rene, who was a few blocks away. "It was a huge horde of people just running away."
Law enforcement officials told NBC News that a small homemade bomb is the preliminary cause of the explosion, and Boston's Police Commissioner said that a third subsequent explosion happened at JFK Library. The police later said via Twitter that the JFK library incident appears to be fire related.
Federal officials told NBC News that Boston police were guarding a "possible suspect" who had been wounded in the blasts, but they cautioned that there was no information at the federal level to consider that person a suspect. Other people were being questioned, law enforcement officials said.
A third, undetonated device was found near the finish line, a House Homeland Security Committee official and three law enforcement officials told NBC News. Authorities also reported an explosion at the John F. Kennedy presidential library, elsewhere in the city, more than an hour after the blasts, but police said that it appeared to be caused by a fire. The police commissioner urged people to stay inside.
Hospitals reported that at least two children were among the injured. Dr. Alisdair Conn, chief of emergency medicine at Massachusetts General Hospital, characterized the wounds as something Americans are more accustomed to seeing on the news from a military-style bombing in Iraq or Israel.
The police said the area around crime scene will be closed for the foreseeable future.
(Read More: NYC, DC Increase Security in Wake of Explosions)
Serghino Rene, 30, who works in alumni relations, was volunteering near the finish line not long before the bombing but was a few blocks away when he heard the explosion.
"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," he said. "We just got away from the scene and away from tall buildings. I'm scared and emotional. I'm wondering if there are people who I was volunteering with who were still there and got hurt."
A spokesman for the Boston Marathon told reporters that no one would be allowed in or out of the hotel that was serving as headquarters for the event.
Suspicious packages were found after the blasts at three Boston subway stops, and authorities were investigating. New York police deployed extra security to landmarks, Pennsylvania Avenue in front of the White House was closed to foot traffic, and the Pentagon tightened security. Federal authorities briefly grounded flights at the Boston airport as a precaution.
"People shouldn't jump to conclusions but we will get to the bottom of this and find out who did this and why they did this," President Obama said in a televised address. "Any responsible individuals, any responsible groups will feel the full weight of justice."
(Read More: What Clues Should Law Enforcement Look For?)
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 elite men began running 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.
Authorities gave a phone number for people in search of loved ones — 617-635-4520. They encouraged people with information about the blasts to call 1-800-494-TIPS.