Honest truth?
These kids were complete morons. It is nearly impossible to have a propane tank blow up, especially from 2 inexperienced little kids with really crappy guns. They were obviously not right in the head. Their plan was a total fail from the start. Those propane bombs would have never blown. Ever.
Good riddance to those clowns. They took unfortunately 13 innocent lives that day. May they rest in piece. Eric and Dylan? May they rot in pieces.