(Newser) – Authorities say a 24-year-old woman found dead a Nevada spa earlier this week had spent at least 10 hours inside a cryochamber—a machine designed to expose users to temperatures as low as negative-240 degrees, KSNV reports. According to experts, no one should spend more than three minutes in one. Chelsea Ake, the manager of the Rejuvenice spa, was closing up by herself when she got into the machine and didn't turn it off, her friend Shae-Lynn Bee tells KSNV. "There are no cameras in there,” she says. “Basically, the only person that does know what happened is Chelsea." Police are blaming "operator error," and say Ake may have suffocated. Her family says she froze to death inside the cryochamber. The machines are meant to expose users to extremely cold air as a substitute for cold water immersion or ice packs. (Read more death stories.)