The parents of Tyler Hilinksi said their deceased son had the degenerative brain disease chronic traumatic encephalopathy (CTE) when he committed suicide in January.
"The medical examiner said he had the brain of a 65-year-old, which is really hard to take,'' Hilinski's father, Mark, told NBC News' Hoda Kotb. "He was the sweetest, most outgoing, giving kid. That was difficult to hear."
Hilinski was a quarterback at Washington State when he shot himself in the head. He was in line to become the Cougars' full-time starter in 2018, and was less than a month removed from a 272-yard passing performance in the Holiday Bowl when he took his own life.
In a documentary about their son's death for Sports Illustrated, Hilinski's parents revealed their son was diagnosed with CTE postmortem by doctors at the Mayo Clinic in Minnesota.
"They said the tau protein was something you would never see in someone who was 21 years old, but in a much more older, elderly man,'' Hilinksi's mother, Kym, said in the documentary. "And it was shocking, because we know Tyler. Yes, he was quiet. Yes, he was a little bit more reserved, but he was always happy."
Hilinski's younger brother, Ryan, is a quarterback in high school, and one of the top prospects in the 2019 recruiting class. He's already committed to play at South Carolina, and said he plans to continue playing football despite the tragic story of his brother.
There is still no cure for CTE, and the disease can only be diagnosed after death - doctors have been working on solutions for both over the last several years.