A senior U.S. intelligence official said two more explosive devices were found near the area, and they were being dismantled.
The first blasts happened at about 2:50 p.m. near the intersection of Boylston and Exeter streets. Bloody spectators were carried away from the area, and witnesses said several victims lost limbs.
"People started throwing down the barricade and running over one another. I literally saw the garbage barrel explode, and I ran as fast as I could," said one runner.
Boston police said two people died, and at least 22 were injured. Many tired and scared runners were seen wandering around the area, desperate to find loved ones.
A doctor at Massachusetts General Hospital said the injured looked as if they "were in a war zone."