Two explosions went off near the finish line of the Boston Marathon on Monday, killing at least two people and injuring dozens more, as runners completed the course and thousands cheered them on. Race organizers described the blasts as bombs.

Authorities reported a third explosion at the John F. Kennedy Library, elsewhere in the city, more than an hour after the blasts. There were no immediate reports of injuries in that explosion. Authorities said that they were investigating and treating it as related, and the police commissioner urged people in Boston to stay inside.

Video from the finish line at the marathon showed people screaming 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.

“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.”

A third, undetonated device was found near the finish line, a House Homeland Security Committee official and three law enforcement officials told NBC News.

Police said at least 23 people had been injured, but the count from hospitals was much higher. Massachusetts General Hospital said that it had 19 patients, Tufts Medical Center nine and Brigham and Women’s Hospital 18 to 20 — a total of 46 to 48.