The Dyatlov Pass Incident

...an event that resulted in the deaths of nine ski hikers in the northern Ural mountains on the night of February 2, 1959.

Six of the group members died of hypothermia and three of fatal injuries.
There were no indications of other people nearby apart from the nine travelers on Kholat Syakhl, nor anyone in the surrounding areas.
The tent had been ripped open from within.
The victims had died 6 to 8 hours after their last meal.
Traces from the camp showed that all group members left the camp of their own accord, on foot.
To dispel the theory of an attack by the indigenous Mansi people, Dr. Boris Vozrozhdenny stated that the fatal injuries of the three bodies could not have been caused by another human being, "because the force of the blows had been too strong and no soft tissue had been damaged".
Forensic radiation tests had shown high doses of radioactive contamination on the clothes of a few victims.

The final verdict was that the group members all died because of a "compelling unknown force". The inquest ceased officially in May 1959 due to the "absence of a guilty party". The files were sent to a secret archive, and the photocopies of the case became available only in the 1990s, with some parts missing.

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And click here to see it on Ancient Aliens (at the 10:22 mark).