Russian police have reopened a 60-year-old investigation into the mysterious deaths of nine students on a cross-country ski trip, in a case which has baffled authorities and sparked conspiracy theories for decades.

Key points: Prosecutors say relatives deserve to know what happened to the nine students who died trekking the Ural Mountains

Prosecutors say relatives deserve to know what happened to the nine students who died trekking the Ural Mountains One skier was found missing a tongue, while two others had fractured skulls

One skier was found missing a tongue, while two others had fractured skulls A 1959 investigation ruled there were no signs of struggle and closed the case

Known as the Dyatlov Pass Incident, the mystery includes a sliced open tent, bodies dressed in underwear in the snow, traces of radiation and a victim missing a tongue.

The deaths have been blamed on everything from avalanches to UFOs, but the Russian prosecutor general's office now hopes to put any far-flung theories to rest.

In an announcement last week, marking the anniversary of the deaths, the official representative of Russia's prosecutor Alexander Kurennoi said it was not a criminal investigation. He acknowledged it was a baffling mystery.

"All of [the deaths] are somehow connected with natural phenomena," he said.

"Relatives, the media and the public still ask prosecutors to determine the truth and don't hide their suspicions that something was hidden from them."



What happened in the mountains?

The story began with a cross-country skiing expedition led by fifth-year Ural Polytechnic Institute student Igor Dyatlov.

Accompanied by six men and two women, he set off on a 300-kilometre trek across the Ural Mountains in late January 1959. The nine hoped to finish the journey in 16 days.

They had planned to telegram their friend Yuri Yudin, a 10th member of the expedition who pulled out of the trip due to injuries, when they reached the village of Vizhai.

Days passed and the telegram never came.

Investigators concluded the tent was slashed from the inside. ( Wikimedia Commons )

A search party was launched on February 20, 1959, with investigators discovering the group's tent, which appeared to have been sliced open from the inside.

Footprints leading from the tent suggested the group were either barefoot or wearing socks.

One woman was found with her tongue removed

The bodies of two men were found at the base of a pine tree about 1.5 kilometres away, stripped down to their underwear and lying beside the remains of a small campfire.

Damage to the branches of a pine about five metres high suggested one of the men tried to climb the tree.

Later, three more bodies were found between the tree and the tent, reportedly positioned in way that suggested they were attempting to return to the campsite.

It took investigators two months to find the remaining four bodies, partially dressed, in a ravine about 75 metres from the pine tree.

An autopsy report noted that one of these four, a 20-year-old woman, was missing her tongue.

She and another victim had fractured ribs, and two had fractured skulls, however a report from the regional prosecutors office, written in May 1959, ruled out foul play.

It deemed three of the four found in the ravine died from injuries, while the other six victims died as the result of exposure to cold.

The initial report concluded there was no evidence of foul play.

"Given the absence of external bodily injuries and signs of struggle on all corpses, the presence of all group values, and taking into account the forensic medical examination of the causes of death of tourists, it should be considered that the cause of the death of tourists was spontaneous force, which the tourists were unable to overcome," a translated version of the report said.

The report ordered an end to the investigation.

According to a 2013 obituary for Yudin published in the UK's Telegraph newspaper, small traces of radiation were found on some of the victims' clothing.

The case files were sealed and the lack of explanation for the deaths fuelled conspiracy theories, with some claiming the Soviet government was involved in the incident.

Other theories have included yetis, UFOs and an attack by local native tribes.

The mystery has featured in podcasts such as Stuff You Missed in History Class, and inspired the film Devil's Pass.



Soviet-era cover-up 'out of the question'

At a press conference in Russia, Sverdlovsk chief of justice Andrei Kuryakov presented journalists with a thick book containing previously classified documents relating to the case.

According to Russian masthead Tass, he assured reporters the government had nothing to do with the deaths.

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He said he was aware of 75 theories, but only a handful stacked up.

"A large part of these 75 versions stem from conspiracy theories this or that way, alleging that the entire incident was engineered by the authorities," Mr Kuryakov said.

"It is absolutely out of question and we have proved that 15 theories explaining the hikers' deaths by secret activities of law enforcement agencies are ungrounded."

A modern investigation

Prosecutors will now approach the case with forensic techniques not available to investigators at the time, Mr Kuryakov said.

"There is information about radiation on the clothes of tourists in the materials of the case, but we will appoint a second special examination," he said.

"We will check everything on modern technology.

"Concerning exhumation, we will order a medical examination if necessary."

According to a report from Russian newspaper Gazeta, the investigation will see a team return to the site in March to recreate the scene of the deaths.