Latest news report from CBS, interviewing the Mayor of Birmingham, is that there is debris on a couple of houses along the flight path. Judging from photos and location of the wreckage, that would seem to indicate to me that debris was pre-crash, rather than a result of the post crash break up.
Does that imply a package blew up on descent and a mid-air breakup?
They reported a couple of loud booms, but that could be the plane "landing"