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CORONADO, Calif. (KSWB) – The U.S. Navy has confirmed the identity of a SEAL candidate who died Friday after completing the grueling training phase commonly known as “Hell Week.”

Seaman Kyle Mullen, a 24-year-old from Manalapan, New Jersey, died at Sharp Coronado Hospital just after 5:40 p.m. on Feb. 4, military officials said in a statement. Earlier that day, he had completed a stage of his Basic Underwater Demolition SEAL class (abbreviated to BUD/S).

Officials say Mullen was not actively training at the time of his death, but started experiencing some kind of medical emergency a short time after finishing. The Navy did not immediately provide specific details about Mullen’s symptoms or what might have caused them. An investigation is now underway.

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

A second candidate from the program was also hospitalized with undisclosed medical issues after Hell Week concluded, officials said. They were taken to Naval Medical Center San Diego in Balboa Park and remained in treatment Saturday. Nexstar’s KSWB has reached out for an update on their status.

Navy SEALs are among the U.S. military’s most elite forces, conducting special operations in harsh environments around the world. Becoming a SEAL is notoriously challenging, especially during Hell Week, which occurs during the first phase of training.

Candidates only get around four hours to sleep during a rigorous five-and-a-half day stretch of physically and mentally exhausting drills. At least half of the candidates don’t make it through, according to the Associated Press.

“It is a miserable time, it is well-named,” Paul Anderson, a retired Navy Seal chief, said. “It’s a gut check … 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.”

Anderson didn’t want to speculate specifically on what happened to the two SEAL candidates this week, but said injuries, sickness and even death are not unheard of during the brutal stretch of training.

The veteran said he’s heard of candidates getting pneumonia or inhaling some water during underwater drills. He also said candidates face immense pressure to cover up any symptoms of illness, because it can prompt them to get held back from the class and forced to repeat the training or give up.

The last death reported during SEAL training came in May 2016, when 21-year-old Seaman James Derek Lovelace died while undergoing a BUD/S pool exercise at Naval Amphibious Base Coronado. The medical examiner ruled his death a homicide by drowning, but no criminal charges were filed.

KSWB’s Megan Healy contributed to this report.