(NewsNation) — Kyle Mullen was 24 years old and in the best shape of his life when he decided to join the Navy SEALs. It was a decision that would prove to be fatal.

Mullen’s mother Regina begged Kyle, a former captain of the Yale football team, not to join. She worried for his safety, fearful he could be injured or killed in combat. Regina had no inkling it would be SEAL training that would kill her son, a death that sparked an internal Navy investigation.

“He was happy. He loved his friends and family and he just wanted to do the right thing and I begged him not to go but he just wanted to save people,” Mullen said. “I was afraid for something to happen to him. I was really worried.”

Kyle had made it through “Hell Week,” a five-day gauntlet of extreme physical and mental conditioning all SEAL candidates must complete. Kyle called his mother on the phone after Hell Week, and she knew something was wrong with her son. He was out of breath on the phone and there was crackling in his lungs when he inhaled.

Soon after that phone call, Regina had a knock on her door. It was Navy officials there to tell her that her son was dead. She told them then and there; it was the Navy that killed him.

“They didn’t treat him medically, he was out of breath when I was on the phone with him and I thought he was in a hospital I had no idea where he was and I didn’t want to keep calling,” Mullen said. “I found out now, he had pneumonia.”

The Navy had tested Kyle’s blood oxygen level earlier that day, Regina said; he measured at just 84%. A healthy level is considered between 95 and 100%. Kyle was given oxygen for five minutes and his blood oxygen level rose to 94%. An hour and a half later, Regina says, the Navy checked his oxygen levels again; they were back down to 86%.

Kyle was not taken to a hospital. He later died.

“I found what I thought were a lot of lies or documentation that didn’t make sense to me and I proved it to them that night,” Mullen said. “It’s impossible on Wednesday for him to be O.K. and fit for duty.”

Retired Navy SEAL Eric Deming said this type of behavior is not uncommon in the program, where performance enhancing drugs and bravado can often permeate the ranks.

“There’s a corrupted cell within the SEAL teams and even our highest (leaders) have said that we have a problem,” Deming said. “But nothing seems to be getting done about the problem.”

Performance enhancing drugs were found in Kyle’s car after his death and are likely used by many other SEAL candidates, Deming said. But the issue of performance enhancing drugs and toxicity within the SEALs is not the fault of candidates, he says, adding that it starts at the top.

“I don’t think they decided on their own to do this. I think it was condoned and pushed and that’s the first way that they start to corrupt somebody within the teams,” Deming said.

A 300-page report was filed by the Navy after it concluded its investigation into Kyle Mullen’s death. Nowhere in those pages did Regina find justice for her son.

“Unless someone is accountable, it’s not good enough,” Mullen said.