A Navy SEAL candidate died Friday and a second was hospitalized in San Diego after completing what is known as Hell Week, an initial, grueling phase of the training for one of America’s most elite military units.

NBC News is withholding the name of the deceased trainee because the Navy said the family notification process is still ongoing. In a statement, Naval Special Warfare command said the death is under investigation.

The statement said the two SEAL candidates were taken to the hospital Friday “several hours after their Basic Underwater Demolition SEAL (BUD/S) class successfully completed Hell Week, part of the first phase of the Navy SEAL assessment and selection pathway. One candidate died at Sharp Coronado Hospital in Coronado, California, on Feb. 4. The other candidate is in stable condition at Naval Medical Center San Diego.”

The sailors were not actively training when they reported symptoms and were transported to receive emergency care, the statement said.

During Hell Week, SEALS candidates participate in a series of high intensity training exercises while being given very little opportunity to sleep.

The exact number of sailors who have died while training to be SEALs was not immediately available, but deaths during BUD/S are not unheard of.

In 2016, NBC News reported extensively on the death of Derek Lovelace, who drowned during a pool exercise in what the Navy ruled was an accident.

Navy records showed that he was the fifth trainee in four months to lose consciousness during a pool exercise.

The Navy says that only about one in five trainees make it through the arduous course, which produces around 200 to 250 SEALs a year.

As many as 17 SEALs who made it through the course died in training accidents over the last two decades, according to news reports, underscoring the dangerous nature of the work, even leaving aside risky combat deployments.