Demaryius Thomas had the degenerative brain disease known as chronic traumatic encephalopathy or CTE, which is linked to repeated blows to the head, according to an announcement Tuesday by doctors from Boston University who had been studying the former NFL star's brain through the Concussion Legacy Foundation (CLF).

Thomas was found dead in his Roswell, Georgia, home this past Dec. 9, just 15 days before his 34th birthday. At the time, his family believed that seizures, which Thomas had battled since a 2019 car crash, might have led to his death. The coroner's office in Fulton County, Georgia, has not yet ruled on the cause of death.

Neuropathologist Dr. Ann McKee, who was part of the Boston University research team, said Thomas most likely died after having a seizure -- a condition not generally associated with CTE.

"CTE itself does not cause death. You don't die from CTE," McKee told ABC News. "What CTE does is it changes your behavior and your personality."

Family members say that in addition to the seizures, which began in 2020, Thomas also struggled with what have come to be known as common conditions associated with CTE: memory loss, paranoia and other erratic behavior, especially in the last year before he died.

"His mood would change, and he would also isolate himself sometimes," Thomas' mother, Katina Stuckey Smith, told ABC News. "He was, like, 'Mom, I don't know what's going on with my body. You know, I gotta get myself together,' and he said, 'I don't feel like myself anymore.'"

Doctors at Boston University said Thomas had stage 2 CTE, which is associated with progressive behavior, cognitive and mood abnormalities. Thomas' relatives say he developed depression and anxiety and had panic attacks in the years before his death. Stage 4 is the most severe state of CTE and is usually associated with dementia.

"I hope this is a wake-up call to high-profile current and former NFL players that CTE is rampant among them, and they need to get involved in creating real solutions," said Dr. Chris Nowinski, co-founder and CEO of the CLF. "CTE should be their No. 1 off-the-field issue."

Thomas is one of more than 300 former NFL players who have been diagnosed with CTE by McKee and the BU CTE Center research team.

A first-round pick in 2010 out of Georgia Tech, Thomas played 10 seasons in the NFL. He spent 8½ seasons with the Denver Broncos, with whom he won two AFC championships and a Super Bowl. The four-time Pro Bowl receiver also played with the Houston Texans and New York Jets, finishing his career with 724 catches for 9,763 yards and 63 touchdowns.

Thomas retired from the NFL in June 2021.

Information from the Associated Press contributed to this report.