The 2nd explosion was conducted by the bomb squad apparently:
http://www.dallasnews.com/sharedcontent/dws/dn/latestnews/stories/100305dntexsuicide.b6e9d8ae.html
Authorities haven't identified what kind of explosive device was used. Boren initially said a second explosive device was detonated by a bomb squad Saturday night, but later said no other devices had been discovered.
Authorities detonated a backpack with a mild explosive device, but no other explosive materials were found inside, Boren said.
Weird thread...