Dear T-Richard-
no, it wasn't shot down. The hijacker pushed the nose over far enough that the aircraft exceeded its max speed and "fluttered." The tail and/or wings rip off, essentially like a road sign in a hurricane.
As in the case of TWA 800, any explosion would have left telltale shrpanel pockmarks all over the wreckage - and no way would that have been supressed for this long.
Also, the USAF would have first tried to contact the pilots on VHF radio frequencies monitored by many other pilots and ATC controllers - also impossible to suppress for this long.
Jets are slick, heavy pieces of metal with massive amounts of thrust coming out of their engines. Point them at terra firma for more than a few seconds and they start shedding parts.
God bless those pax and crew. May we all show the furious strength that they did.....