Perhaps we had a 'Sky Marshall' onboard and he was able to report in before the crash, indicating an explosion and/or more.
THEY DID NOT SAY THERE WAS A BOMB. I've been listening to FOX and they said NO SUCH THING.
What was said is that there was a SOUND like an explosion prior to the aircraft augering in. This can come from any number of things, only one of which is a bomb.