How can we use electronics to detect explosives?

Lets say we have to build an anti-bomb security system so that any bomb placed in public places by terrorists can be automatically detected and defused quickly and effectively. We could have a lot of insect-like bots in our atmosphere who will form a grid all over the world and detect any explosive device and then defuse it.
How do we do the first major task of electronically detecting an explosive in a crowded place?