SAN DIEGO (NEXSTAR) – A candidate for the Navy SEALs has died, and another has been hospitalized in San Diego, after completing a training phase commonly referred to as Hell Week, a representative for the Navy confirmed to the Associated Press.

The Naval Special Warfare Command said the two had already successfully completed the first phase of BUD/S (basic underwater demolition/SEAL) training several hours before they were hospitalized, and were not actively training when they first required emergency medical attention.

One of the candidates died at Sharp Coronado Hospital on Feb. 4. The other was said to be in stable condition at Naval Medical Center San Diego in Balboa Park, the Navy said.

Naval Special Warfare Command has launched an investigation into the incidents.

Hell Week, which refers to a specific portion of the first phase of SEAL training, is described as a “grueling five-and-a-half-day stretch” during which candidates complete rigorous physical training on “about four total hours” of sleep, according to a video shared by an official website for the Naval Special Warfare Command.

The program is said to be so grueling that at least half of the candidates don’t make it through, the AP reports.

Rep. Scott Peters, from California’s 52nd congressional district, released a statement regarding the incident.

“My prayers are with the family and loved ones of the deceased during this very difficult time, and I wish for a full recovery of the other candidate reported injured yesterday. These courageous, talented sailors were on their way to becoming some of our country’s most elite military personnel. We must find out what transpired so we can do whatever we can to keep others safe. In the days and weeks ahead, I look forward to learning more as the Navy conducts a thorough investigation.”

In 2016, a U.S. Navy SEAL trainee died in a swimming exercise during his first week of BUD/S in Coronado, according to military officials. Seaman James Derek Lovelace, 21, was participating in a swimming exercise when a safety observer noticed he was having difficulty, the military said in a release. He was brought to the edge of the pool where he lost consciousness. Lovelace was taken to a hospital where he died.

The Associated Press contributed to this report.