Boston police commissioner: 3rd explosion at JFK Library.

In this image from video provided by WBZ TV, spectators and runners run from what was described as twin explosions that shook the finish line of the Boston Marathon, Monday, April 15, 2013, in Boston. Two explosions shattered the euphoria of the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/WBZTV)

BOSTON (AP) — Boston police commissioner: 3rd explosion at JFK Library.