In this image from video provided by Ryan Hoyme, the second explosion can be seen in the distance as smoke from the first explosion surrounds spectators exiting the stands during the Boston Marathon in Boston, Monday, April 15, 2013. (AP Photo/Ryan Hoyme)
Gallery: More photos from Boston explosion
Apr. 16, 2013 @ 01:16 PM
Investigators believe the two bombs that ripped through the crowds at the Boston Marathon, killing three people and wounding more than 170, were fashioned out of pressure cookers and packed with shards of metal, nails and ball bearings.