The U.S. Navy continues to investigate the death of 24-year-old SEAL candidate Kyle Mullen and the illness of a fellow SEAL candidate last Friday after both sailors successfully completed the arduous “Hell Week.”

Mullen is the fourth SEAL candidate to die during SEAL selection since 2001. The death of seaman Derek Lovelace in 2016 during a swimming exercise led to changes that aimed to increase instructor awareness of sailors who might be in physical distress. That included reducing the number of candidates who could be in a pool during swimming exercises.

“One such accident is one too many,” Pentagon spokesman John Kirby told reporters on Monday. “We just don’t know what happened.”

Mullen, a New Jersey native, was hailed Monday by New Jersey Gov. Phil Murphy as “a living legend” because of his athletic prowess in high school and collegiate football. Murphy ordered flags in the state to be flown at half-staff to honor Mullen, who joined the Navy 10 months ago on an enlistment contract to specifically pursue becoming an elite Navy SEAL.

Kirby urged patience as the Navy conducts its investigation and said it was premature to criticize the SEAL selection process.

“The training has to be demanding, given the work that our Navy SEALs do on behalf of this country every single day,” Kirby said. “So you would expect the standards to be very, very high for their readiness.”

Conducted during the third week of the course, “Hell Week” pushes SEAL candidates to their physical and mental limits with a series of constant physical tests during a 120-hour stretch when they only receive between two to four hours of sleep.

“Hell Week really is one week of a simulated combat environment,” said Cpt. Duncan Smith, a retired SEAL who served 32 years in the Navy and is now the executive director of the SEAL Family Foundation. “It’s physical. It’s also mental and it’s also our early look at how people operate as a member of a team,” he added.

“It’s the thing that a lot of people decide, makes them choose to not be a SEAL anymore,” Smith said.

More than half of the SEAL candidates who enter “Hell Week” drop out at some point as they carry out long-distance swims in the ocean in uniform, carry heavy inflatable boats and run a combined 200 miles throughout the week.

“There’s nothing about Hell Week that’s meant to be abusive,” said Smith. “It’s demanding but there is a tremendous amount of science that goes into it.”

“There are medical professionals there every step of the way,” he added. “These are some of the most studied individuals medically with a goal of keeping them alive and healthy and strong.”

During the few hours of sleep that the SEAL candidates are allowed during the week, they are monitored individually by medical professionals, according to Eric Oehlerich, a retired SEAL and ABC News contributor.

“SEAL training takes you beyond your personal limits,” Oehlerich said. “It’s designed to push you beyond your perception of what’s possible, breaking glass ceilings of what you’re capable of both mentally and physically.”

“Adhering to the training curriculum keeps SEALs alive in combat,” he said.