FoxNews - 291 on board.
KTVU Live said that the plane tail section hit a retainer wall at the end of the runway. That ripped off a section as the plane continued to slide down the runway.
RT [Russia Today] News had a good shot of the rip-rap [sea wall]. Large rocks and boulders scattered over a 50-yard area. Indicative of a strike.
If plane came in too low and the tail clipped the rip-rap [breaking off], it could have pitched the nose of the fuselage downward - starting the flips and cartwheels that have been reported.
FYI: iPhone pic of passengers disembarking the plane after the crash shows no fire in cabin immediately after impact. Pic shows people running away with their carry-ons ...