That they were in a militarily restricted zone which Soviet forces used to test weapons has absolutely nothing to do with it ... Penalty for intrusion into such zones was death on the spot.
Read the book. This was an open area in which Russian youths who wanted their ski training licenses HAD to go. It was part of their training.
Also one member of the group “turned back.” To me he is a prime suspect. Some of the bodies were mutilated. One had his eyes gouged out.