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Months after the 33-year-old’s unexpected death, Demaryius Thomas’ family has revealed that the NFL star’s brain showed signs of a degenerative brain disease.

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Thomas’ parents, Katrina Smith and Bobby Thomas, donated their son’s brain to Boston University, where researchers found that Thomas suffered from stage 2 CTE and that his death was caused by a separate seizure disorder, leading to cardiac arrest, ABC News reported.

Chronic traumatic encephalopathy, or CTE as it is more commonly known, is a degeneration of the brain believed to be the result of repetitive brain trauma, according to Boston University’s CTE Center. CTE is often seen in athletes, with changes in the brain appearing “months, years, or even decades after the last brain trauma or end of active athletic involvement.” While symptoms associated with CTE include memory loss, impaired judgement, impulse control, depression and dementia, the only conclusive diagnosis of the disease can be made after death with an analysis of the brain.

Thomas died after he was found unresponsive in his shower on Dec. 9, 2021. A police report obtained by WSB-TV showed Thomas’ cause of death listed as cardiac arrest.

The seizure disorder that led to Thomas’ death was not from CTE, but from a 2019 car crash, The New York Times reported. Seizures had previously caused Thomas to fall down stairs and wreck cars.

The director of Neuropathology Core at Boston University, Dr. Ann McKee, told ABC News that she has diagnosed CTE in 600-700 athletes after death, with the first cases being identified in the brains of former NFL players.

“CTE itself does not cause death,” McKee told ABC News. “You don’t die from CTE. What CTE does is it changes your behavior and your personality.” Describing Thomas’ brain, McKee told ABC, “We found what we’ve seen in so many other players under the age of 34. On the basis of multiple lesions in the frontal lobes and temporal lobe are beginning degeneration of deeper areas of the brain, he was diagnosed with CTE, stage 2.”

Thomas grew up in Georgia, playing high school football for West Laurens High school before signing to play college football at Georgia Tech, WSB-TV reported. He went on to play for the NFL for 10 years, including eight years with the Denver Broncos and a Super Bowl win.

Thomas’ parents told ABC News that they saw changes in their son that they believe may have been a result of CTE.

“When they started explaining – side effects, warning signs and that’s when the bell started ringing. When they said it, I was like, ‘Man he was doing that. He did that, too,’” Bobby Thomas told ABC News. “And I faulted myself, cause I said, ‘I know I coulda done more.’”