Tragedy and a troubling mystery have enveloped Mount Everest as the mountain’s climbing season reaches its peak. The bodies of three Indian climbers were retrieved Sunday amid reports that climbers were being jeopardized by the disappearance of oxygen bottles. The two situations have not been linked at this time.

The bodies of Ravi Kumar, who died last weekend, and Paresh Chandra Nath and Gautam Ghosh, who died last year on the mountain, were brought down after being recovered by Sherpa guides near the summit. They were taken by a helicopter from a camp at a lower elevation to Kathmandu for autopsy.

The window for reaching the summit closes as May ends and conditions deteriorate. With the possibility of a successful climb becoming slimmer, there are reports that oxygen is disappearing.

“It is becoming a serious issue up there,” mountain guide Nima Tenji Sherpa told the BBC on Friday.

“I kept on hearing from expedition groups that their oxygen bottles had disappeared and that could be life-threatening – particularly when they have used up what they are carrying on their way up and they are still not on the summit yet, or they plan to use the stocked bottles on their way back,” added Tenji Sherpa, who had just returned from Everest.

While it’s possible to summit Everest without oxygen, it’s not recommended for most climbers because of the mountain’s extreme elevation. Soaring some 5.5 miles above sea level, Everest’s air at its peak can’t sustain life for more than a few hours. Without extensive training, lack of oxygen can bring on serious frostbite, as well as a condition called hypoxia that affects the brain, causing headaches, hallucinations and eventually death.

The first group of climbers summited the mountain on May 15 and it didn’t take long for reports of the suspected thefts to come in.

This isn’t the first year climbers have seen their oxygen go missing. The problem has become so commonplace this year that the Nepal National Mountain Guides Association called it “a trend.”