ISIS have released a video now which would seem to support the claim that they took it down. Very tragic.
So far it appears to be a hoax. ISIS in the Sinai would not have a means to down the aircraft from its high altitude unless an onboard bomb had been used. Investigators so far have indicated the wreckage shows no sign of such an explosion, and the pilot radioed an emergency call citing technical difficulties that is usually inconsistent with a catastrophic explosion.