Nine experienced cross-country skiers hurriedly left their tent on a Urals slope in the middle of the night, casting aside skis, food and their warm coats.

Clad in their sleepwear, the young people dashed headlong down a snowy slope toward a thick forest, where they stood no chance of surviving bitter temperatures of around minus 30 degrees Celsius.

Baffled investigators said the group died as a result of "a compelling unknown force" -- and then abruptly closed the case and filed it as top secret.

The deaths, which occurred 49 years ago on Saturday, remain one of the deepest mysteries in the Urals. Records related to the incident were unsealed in the early 1990s, but friends of those who died are still searching for answers.

"If I had a chance to ask God just one question, it would be, 'What really happened to my friends that night?'" said Yury Yudin, the only member of the skiing expedition who survived.

Yudin and nine other students from the Ural Polytechnic Institute embarked on the skiing expedition to Otorten Mountain in the northern Urals on Jan. 28, 1959. Yudin fell ill near Vizhai, the last settlement before the mountain, and was left behind.

What happened next has been reconstructed from the diaries of the rest of the group and the photographs they took. Copies of the diaries, photos and investigators' records were reviewed for this article.

The skiers, led by Igor Dyatlov, 23, set up camp for the night of Feb. 2 on the slope of Kholat-Syakhl, a mountain next to Otorten. They pitched their tents at around 5:00 p.m., investigators said, citing photos that they developed from rolls of film found among the abandoned belongings.

Why the nine skiers picked the spot is unclear. The group could have detoured just 1.5 kilometers down the mountain to a forest, where they would have found shelter from the harsh elements.

"Dyatlov probably did not want to lose the distance they had covered, or he decided to practice camping on the mountain slope," Yudin said by telephone from Solikamsk, a town near Yekaterinburg, where the institute, now named Ural State Technical University, is located.

When the group left the institute for the expedition, Dyatlov promised to send a telegram as soon as they returned to Vizhai from Otorten Mountain, which he said would be by Feb. 12.

But Yudin said Dyatlov told him when they parted ways that the group would probably return a few days later than planned.

As such, no one was worried when the group failed to reappear on Feb. 12.

Only on Feb. 20, after relatives raised the alarm, did the institute send out a search-and-rescue team of teachers and students. The police and army dispatched their airplanes and helicopters later.

Puzzling Evidence

The volunteer rescuers found the abandoned camp on Feb. 26.

"We discovered that the tent was half torn down and covered with snow. It was empty, and all the group's belongings and shoes had been left behind," Mikhail Sharavin, the student who found the tent, said by telephone from Yekaterinburg.

Investigators said the tent had been cut open from inside and counted traces of footprints from eight or nine people in meter-deep snow. The footprints had been left by people who were wearing socks, a single shoe or were barefoot.

Investigators matched the footprints to the members of the group, saying there was no evidence of a struggle or that other people had entered the camp.

The footsteps led down the slope toward the forest but disappeared after 500 meters.

Sharavin found the first two bodies at the edge of the forest, under a towering pine tree. The two -- Georgy Krivonischenko, 24, and Yury Doroshenko, 21, were barefoot and dressed in their underclothes.

Charred remains of a fire lay nearby. The branches on the tree were broken up to five meters high, suggesting that a skier had climbed up to look for something, perhaps the camp, Sharavin said. Broken branches also were scattered on the snow.

The next three bodies -- Dyatlov, Zina Kolmogorova, 22, and Rustem Slobodin, 23 -- were found between the tree and the camp. The way the bodies were lying indicated that the three had been trying to return to the camp.

The authorities immediately opened a criminal investigation, but autopsies failed to find evidence of foul play. Doctors said the five had died of hypothermia. Slobodin's skull was fractured, but the injury was not considered fatal.

It took two months to locate the remaining skiers. Their bodies were found buried under four meters of snow in a forest ravine, 75 meters away from the pine tree. The four -- Nicolas Thibeaux-Brignollel, 24, Ludmila Dubinina, 21, Alexander Zolotaryov, 37, and Alexander Kolevatov, 25 -- appeared to have suffered traumatic deaths. Thibeaux-Brignollel's skull had been crushed, and Dubunina and Zolotarev had numerous broken ribs. Dubinina also had no tongue.

The bodies, however, showed no external wounds.

The four were better dressed than the rest, and those who had died first had apparently relinquished their clothes to the others. Zolotaryov was wearing Dubinina's faux fur coat and hat, while Dubinina's foot was wrapped in a piece of Krivonishenko's wool pants.

Deepening the mystery, a test of the clothes found they contained high levels of radiation.

The investigation, however, was closed after a few months, and investigators said they could not find anyone to accuse of wrongdoing. Case files were sent to a secret archive. Skiers and other adventurers were barred from the area for three years.

"I was 12 at that time, but I do remember the deep resonance that the accident had with the public, despite the authorities' efforts to keep relatives and investigators silent," said Yury Kuntsevich, head of the Yekaterinburg-based Dyatlov Foundation, which is trying to unravel the mystery.

Investigators first explored the theory that the local Mansi people had killed the skiers in revenge for trespassing on their land. No evidence, however, was found to back up the theory; Neither Otorten nor Kholat-Syakhl were considered sacred or taboo places by the Mansi, case documents said.

Further debunking the theory, a doctor who examined the bodies in 1959 said he believed that no man could have inflicted the injuries because the force of the blows had been too strong and no soft tissue had been damaged,

"It was equal to the effect of a car crash," said the doctor, Boris Vozrozhdenny, according to case documents.

'Bright Flying Spheres'

In 1990, the chief investigator, Lev Ivanov, said in an interview that he had been ordered by senior regional officials to close the case and classify the findings as secret. He said the officials had been worried by reports from multiple eyewitnesses, including the weather service and the military, that "bright flying spheres" had been spotted in the area in February and March 1959.

"I suspected at the time and am almost sure now that these bright flying spheres had a direct connection to the group's death," Ivanov told Leninsky Put, a small Kazakh newspaper. He retired in Kazakhstan and has since died.

The declassified files contain testimony from the leader of a group of adventurers who camped about 50 kilometers south of the skiers on the same night. He said his group saw strange orange spheres floating in the night sky in the direction of Kholat-Syakhl.

Ivanov speculated that one skier might have left the tent during the night, seen a sphere and woken up the others with his cries. Ivanov said the sphere might have exploded as they ran toward the forest, killing the four who had serious injuries and cracking Slobodin's skull.

Yudin said he also thought an explosion had killed his friends. He said the level of secrecy surrounding the incident suggests that the group might have inadvertently entered a secret military testing ground. He said the radiation on the clothes supported his theory.

Kuntsevich agreed, saying another clue to the deaths was the fact that the faces of the first five bodies had been inexplicably tan. "I attended the funerals of the first five victims and remember that their faces look liked they had a deep brown tan," he said.

Yudin also said the released documents contained no information about the condition of the skiers' internal organs. "I know for sure that there were special boxes with their organs sent for examination, " he said.

No traces of an explosion, however, have been found near Kholat-Syakhl.

No Records of Missiles

While a missile fired from the Baikonur Cosmodrome in Kazakhstan could have reached the northern Urals, there are no records of any launches at the time, said Alexander Zeleznyakov, a historian on Soviet missiles and a senior official with the Korolyov Rocket and Space Corporation Energia. The Soviet Union's other main launch pad, Plesetsk, only opened in late 1959. Zeleznyakov also said the surface-to-air missiles that could have been launched from the pads had not yet been built.

The Defense Ministry and the Yekaterinburg regional prosecutor's office said they had no immediate information, citing the age of the case.

Kuntsevich said he had led a group to the area last year and found a "cemetery" of scrap metal that suggested the military had conducted experiments there at some time.

"We can't say what kind of military technology was tested, but the catastrophe of 1959 was man-made," he said.

Yudin said the military might have found the tent before the volunteer rescuers. He said he had been asked to identify the owner of every object found at the scene and had failed to find a match for a piece of cloth that looked like it had come from a soldier's coat, a pair of glasses, a pair of skis and a piece of a ski.

Yudin also said he had seen documents that led him to believe that the criminal investigation had been opened on Feb. 6, 14 days before the search team found the tent.

Dyatlov's friends have looked into whether the deaths might have been caused by an avalanche. Setting up the camp on the slope might have disturbed the snow above, causing it to tumble down a few hours later. This would explain the ripped tent, which the skiers would have had to cut open to exit.

Skeptics of this theory point out that the skiers left the camp by foot and traveled more than a kilometer in minus 30 C.

Thibeaux-Brignollel would have been unconscious due to his shattered skull, said Mikhail Kornev, a doctor with the S.M. Kirov Russian Medical Military Academy.

But his friends could have carried him. After all, investigators could not decide whether there were eight or nine pairs of footprints in the snow.

Also, Dubinina and Zolotarev could have walked with their broken ribs, Kornev said. "I can grant this possibility since the situation was extreme," he said.

Six former rescuers and 31 independent experts gathered Friday in Yekaterinburg to look for answers about the incident. They concluded that the military had been carrying out tests in the area and had inadvertantly caused the deaths.

But "we still lack documents and ask the Defense Ministry, the space agency and the FSB to provide us with them to obtain a full picture," the participants said in a statement.

The conference was organized by Ural State Technical University, the Dyatlov Foundation and several nongovernmental organizations.

What really happened on the night of Feb. 2, 1959, may never be known. But Dyatlov is unlikely to be forgotten anytime soon. The area where the group set up their last camp has been officially named Dyatlov's Pass.