Dallas Cowboys football player Marshawn Kneeland was suffering from stage 1 chronic traumatic encephalopathy (CTE) when he died by suicide last year, scientists said.

Researchers at Boston University's CTE Centre made the diagnosis through a postmortem brain tissue analysis of the defensive lineman, who died aged 24.

"Unfortunately, I was not surprised to find CTE in the brain of Mr Kneeland, because we have found this progressive brain disease in nearly half of the athletes we've studied who have died before the age of 30," said the centre's director, Dr Ann McKee.

Stage 1 is the mildest of CTE's four stages, where headaches and loss of attention or concentration are common symptoms.

In a statement, Kneeland's family said the diagnosis provided "important context" about some of the struggles he was facing.

"We share this information to help people understand what NFL and other high-contact sport athletes might be struggling with," they said. "We continue to remember Marshawn with compassion for the person he was, rather than defining him by the final moments of his life."

Kneeland died in November 2025, after a car chase with Texas State Troopers who said they tried to pull him over for a traffic violation. Police at the time said he crashed his car and then fled on foot. He was later found dead from a self-inflicted gunshot wound in the early morning hours.

Kneeland was drafted by the Cowboys in the second round in 2024 and played 18 times for the team, including four starts. He started playing tackle football when he was 7 years old and was on Western Michigan University's team before joining the Cowboys as a defensive end.

What is CTE?

Research has shown that athletes such as footballers, rugby players and fighters like boxers have a significantly higher risk of developing CTE than the general population.

CTE can only be diagnosed after death. The disease - which is caused by repeated blows to the head - has been found in the brains of dozens of former National Football League (NFL) players.

In the first stage of the disease, some people also experience short-term memory problems, depression, aggressive tendencies, and executive function issues, according to the CTE Society, a nonprofit group.

In later stages, those problems grow stronger, and others, such as mood swings and suicidal thoughts, emerge. In its final stage, a person can experience severe cognition issues and memory loss, as well as dementia, according to the CTE Society.

Boston University's CTE Center said the cause of suicide is complex and a postmortem CTE diagnosis is not known to be a risk factor for suicide.

But CTE researchers said the case showed not enough has been down to lower the risk of the disease for athletes.

"Mr Kneeland played in the modern era of concussion protocols and better helmets, and yet he still developed CTE," said Dr Chris Nowinski, co-founder and CEO of the Concussion & CTE Foundation. "We have no reason to believe the current generation is at a lower risk of CTE than previous generations."