UPDATE:IV: Boston police are telling CNN that at least two people are dead and 22 wounded. Meanwhile, Boston police have reportedly detonated a third device near the Boston Public Library. (Just increased to 29 injured.)
UPDATE III: Here's a live blog from Boston.com with considerable detail about the explosions. Some runners and spectators suffered horrendous injuries. It appears the explosions happened at street level, leading to speculation that explosions from a trash can. Police in Boston are continuing to search for other possible devices; one was reportedly found at the Mandarin Hotel,
UPDATE II: This "vine" captures the actual explosion.
UPDATE: Fox News is reporting that three people have been killed in the explosions. Here is a video of one of the blasts below: