The explosion at the finish line of the Boston Marathon appears to have been caught on tape by a social media user, using iPhone's new VINE app.
In the video, what appears to be a deliberate detonation can be clearly seen in the middle of the crowd watching runners approaching the finish line from the sidewalk. The blast radius appears to have been quite small, with people just dozens of feet away running from the area unharmed. However, a dense crowd can clearly be seen in the area of the unseen device before the explosion.
Warning: the video is disturbing and graphic. It can be found here.