Forensic Investigators Discover Clues to Boston Bombing

The forensic investigation of the Boston Marathon bombing is developing a preliminary picture of how it was done, though not yet who might have done it.

Almost immediately, the two billowing clouds of white smoke visible in videos were a tipoff to bomb specialists about the type of explosives used. Within 24 hours, enough bomb fragments were retrieved by investigators to suggest that at least one of the two improvised explosive devices was a metal pressure-cooker pot packed with explosives -- a low-tech design that has been promoted by al-Qaeda to its radicalized American followers.