When Noah Green, a 25-year-old with no known history of violence, crashed his car into a barricade at the Capitol building in Washington DC, killing one police officer, before lunging at others with a knife, his own family were grappling for answers.

“My heart just sank,” his mother, Mazie Green, tells me. It was a murder, which, on the face of it, had nothing to do with American football.

But three years on and speaking publicly for the first time since that day, Mazie says she now believes it has everything to do with American football.

Green was shot dead by responding police, and in the days after the killing the FBI recommended that Mazie submit Noah’s brain to be analysed.

The diagnosis came back months later, indicating Green had stage one Chronic Traumatic Encephalopathy, more commonly known as CTE.

It’s a brain disease caused by repetitive blows to the head and it afflicts participants of contact sport, including American football. Symptoms include aggression, paranoia and problems controlling impulses.

“Noah took big hits,” Mazie says. At Alleghany High School in rural Virginia, Green had played in defence and was voted most valuable player and he later played for Christopher Newport University.

Teammates recall him being dependable and good-natured but Mazie says she noticed changes after he suffered several head injuries.

“He wanted to be tough, to prove himself,” Mazie says, “But there were changes. He would start wearing blankets around his head and I thought it was a teenage thing, but it was because he was so sensitive to the light. Then he would lose his keys and he forgot how to cook, prepare his meals.

“After that, he started with these really bad headaches. One day he said ‘I don’t know what’s wrong with me, mum, I’ve lost 20 pounds…. I feel like I need to leave. I’ve got to get out of the country. They’re going to kill me, the FBI, they’re going to kill me.’

“He was paranoid.”

Officer William Evans, the police officer who Noah Green killed, left behind two young children. I ask Mazie if she has any message for his family.

“Officer Evans should not have died that day,” she says, “Noah should not have died that day. Someone has to take the responsibility for telling parents what to do if something’s just not quite right with those kids that are out there playing football for entertainment.”

Shannon Terranova, the former spouse of Officer Evans and mother of his two young children, said: “I want to be mindful of all who are impacted by this real-life horror; but it is difficult for me to comprehend any rationalisation of what happened to Billy and the events that led up to his death. I appreciate the efforts in bringing awareness to the long-term implications of bodily trauma caused by sports injuries. However, nothing can justify what Billy’s co-workers and family experienced, saw, and felt on April 2 2021, and every day since.”

Christopher Newport University declined to comment on Noah Green’s case. Alleghany High School did not respond to Sky News’s request for comment.

The question over whether CTE is linked to violent crime has come to the fore after numerous incidents of violent ex-football players.

Former San Francisco 49ers star Phillip Adams shot dead six people in an explosion of violence in 2021.

He murdered doctor Robert Lesslie, his wife, Barbara Lesslie, and two of their grandchildren, Adah, 9, and Noah, 5 at their home in South Carolina.

He also killed James Lewis and Robert Shook, who were working on an air conditioning unit at the house. Analysis of Adams’ brain showed he had severe CTE.

Kellen Winslow, another former NFL player, was convicted of multiple rapes in 2021.

His lawyer argued for his sentence to be reduced because of what he says was head trauma suffered on the football field. That potential mitigating factor was rejected by a judge.

Most experts say it is hard to say definitively what motivates someone to commit a crime, but the symptoms CTE causes could all contribute. More research into the causes of CTE and what factors might make some people more susceptible is under way.

Sky News was given access to the national sports brain bank in Pittsburgh, Pennsylvania where former professional and amateur American footballers are being urged to donate their brains for study.

Inside the histology laboratory, Dr Julia Kofler slices open a brain with a knife to show me the cross-section.

It is impossible to diagnose CTE with the naked eye so she takes a tiny sample of the brain tissue and loads it onto a slide so it can be analysed under the microscope.

I ask if she thinks there is a link between CTE and violent crime. “It’s really difficult to draw any conclusions about what motivates someone to commit a crime based just on their pathology,” she says, “but we certainly know that neurodegenerative diseases can cause all sorts of different behavioural changes and changes in executive function and judgement, so it certainly could have contributed.”

‘We watched him lose himself’

Karen Kinzle Zegel is one of those fighting for more research. Her son, Patrick Risha, had CTE and died by suicide aged 32. He had played American football throughout his childhood and at university.

“We watched him over 10 years, sadly lose himself, lose his dignity,” she says, “He was paranoid, he was argumentative.

“One time there was an incident with him and he said a homeless guy attacked him in Pittsburgh and he broke his hand punching this person. The rage he had was definitely scary.”

Through her organisation Stop CTE, Karen is campaigning for the brains of those who commit mass violence to be analysed for traumatic injury.

“Every time we’re looking at the symptoms like ‘they lost a job, they, broke up with their girlfriend’.

“Everybody wants to know why, why would someone take another person’s life? But if you’ve dealt with somebody whose brain became unwired you see the lack of empathy. They don’t care about other people, sadly.

“We’re not going back to the root cause, which could be a damaged brain.”

Concerns about brain injuries have contributed to the growth of flag football, a lesser contact sport which means fewer big hits and not an obvious danger.

But the popularity of the NFL as a spectator sport is enduring. Last week’s Super Bowl final was the most-watched TV event in American history.

But for its stars, the damage may already been done. The human cost of the richest sport league in the world is far too difficult to count.

Sky News contacted the NFL for comment about our report. In response, the NFL provided details of the funding they are giving into CTE-related research, mental health support and the physical safety measures they say they are taking in the sport.