Two explosions went off near the finish line of the Boston Marathon on Monday, witnesses told NBC News.

The Associated Press reported that bloody spectators were being carried to a medical tent that had been set up to care for tired runners. Video from the finish line showed screams and an enormous cloud of white smoke, and about 20 seconds between the explosions.

Will Ritter, the spokesman for a Massachusetts Senate candidate, 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.

“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.”

Janet Wu, a reporter for NBC affiliate WHDH, told NBC News that she heard two loud explosions. Jackie Bruno, a reporter for New England Cable News, said on Twitter that she saw people’s legs blown off.

Read it at NBC News.