Kyle Mullen, a 24-year-old candidate for the Navy SEALs died on Friday during the training period known as “Hell Week.” Another candidate is in stable condition after being hospitalized at Naval Medical Center San Diego.

“We extend our deepest sympathies to Seaman Mullen’s family for their loss,” Rear Adm. H.W. Howard III, commander of Naval Special Warfare Command, said in a Navy press release on Sunday. “We are extending every form of support we can to the Mullen family and Kyle’s BUD/S classmates.”

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Mullen’s cause of death has yet to be revealed and an investigation is underway.

The grueling training section involves “basic underwater demolition, survival and other combat tactics,” according to

Retired Navy SEAL chief Paul Anderson detailed the infamous “Hell Week” in a statement provided to Fox 5 San Diego.

“It is a miserable time, it is well-named,” he said. “It’s a gut check. You get approximately four hours of sleep the whole week. You are constantly doing physical evolutions whether it’s physical obstacles, going in and out of the surf zone with your boats, running, you are constantly wet and you are always sandy.”

He continued: “They may have had pneumonia and went home to try and cover it up because if you are sick or injured you can be held back from your class. I think they didn’t want to lose their place and have to go through all this again so they may have tried to hide their symptoms, though again I don’t know.”