A cryotherapy session in Paris has turned deadly, with one woman dying and another hospitalized in critical condition due to a nitrogen leak from a cold chamber, according to French investigators.

The tragedy occurred on Monday evening at a small sports center located in the 11th district of the French capital, a police source told the AFP news agency.

According to initial findings, a nitrogen leak from the cryochamber is thought to have caused the poisoning, a source close to the investigation said. An employee of the gym, who was in her late 20s, died.

A client of the establishment, who is in her 30s, was hospitalized in critical condition.

"An investigation into the cause of death has been launched," the Paris public prosecutor's office said.

Three people who were in contact with the victims and provided first aid also received treatment, the police source said.

The gym, where 150 people were present when the tragedy struck, was evacuated shortly afterwards.

AFP journalists present at the scene on Monday evening saw a body covered with a white sheet being carried out of the sports center.

Screens were set up to conceal what was happening in front of the door to the gym and along the pavement.

What is cryotherapy?
Susan Kwiecien, clinical research manager at Manhattan Eye, Ear and Throat Hospital's Nicholas Institute of Sports Medicine and Athletic Trauma, or NISMAT, previously told CBS News that cryotherapy is the "reduction of tissue temperature by the withdrawal of heat from the body."

To withdraw that heat, you use something cold — whether it's an ice bath or a whole body cryotherapy chamber.

"Ice itself is a type of cryotherapy if you put it on your skin," she said.

Nitrogen is a colorless, odorless gas that is commonly used in cryotherapy.

During a session in a walk-in chamber a person is exposed to temperatures below -100 degrees Celsius for a recommended time of no more than three minutes.

Advocates promote whole body cryotherapy for reducing muscle soreness, stress, rheumatism and various skin conditions. Star athletes and celebrities like Hailey Bieber have used whole body cryotherapy as an alternative to ice packs and cold water baths.

But many experts warn that the treatment has not been proven to be medically sound and are urging further research to determine the short- and long-term effects.

Diego Brisset, 26, said he planned to work out at the gym where the incident took place but was told it was closed on Monday evening.

He said he did not practice cryotherapy.

"I was always told it was dangerous," he said.

Cryotherapy came under scrutiny in the United States in 2015 after a woman froze to death at a Las Vegas spa. The 24-year-old woman was believed to have entered one of the spa's cold chambers after business hours to relieve some aches, and was discovered the next day by a co-worker.

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