ISIS?
Unknown at this point. Despite Russian denial, it could still be a technical fault and airframe failure. If the prior damage to the tail already known about happened to start an undetected fracture in the composites used in the airframe, the fractures could have caused a failure in the elevator system that caused the plane to dive at a speed greater than the maximum specifications allow, resulting in the disintegration of the fuselage in mid-air above the Sinai desert.