Remaining TU-154s in service are so old as to be virtual death traps, anyway. Given the aircraft type, this can’t be assumed as terrorism yet.
Granted that the 154 does not have the greatest safety record.
But no SOS call is ominous
http://www.itv.com/news/story/2016-12-25/russian-military-plane-with-92-on-board-crashes-into-black-sea/