not sure how certain this is....yet.

LONDON (AP) - The bombs that destroyed three London Underground cars and a double-decker bus were each lighter than 10 pounds and could be carried in a backpack, police said Friday. An explosives expert said they were likely crude homemade devices set off with a simple timer.

The bombs were probably made from simple, relatively easy-to-obtain plastic explosives, not the higher-grade military plastics like Semtex that would have killed far more people, said Andy Oppenheimer, a weapons expert who consults for Jane's Information Group.

``Any crook with ready cash could obtain this stuff if they knew where to look for it,'' said Alex Standish, the editor of Jane's Intelligence Digest.