Boston firefighters, right, talk with FBI agents and a crime scene photographer at the scene of Monday's Boston Marathon explosions, which killed at least three and injured more than 140, in Boston, Tuesday, April 16, 2013. The bombs that blew up seconds apart near the finish line left the streets spattered with blood and glass, and gaping questions of who chose to attack at the Boston Marathon and why. (AP Photo/Charles Krupa)
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.