Coaches at Harrisonville High School, in Missouri, still have the tape of Michael Keck’s first football game. The freshman starter — powerfully built and preternaturally talented for someone so young — barrels toward an opposing player, tackling him so hard that pieces of his helmet fly off. The play is set to a song that goes, “Here comes the boom,” and the tape is worn from watching and rewatching. It is a relic of a hometown hero, the handsome young star who willed himself out of a troubled upbringing and into a starting spot on the University of Missouri football team.

But it’s also a portent of the hundreds, thousands, of tackles that would come. Here would come the boom again and again. All through four years of high school and two more of college, not to mention the decade he spent on youth teams, back when a regulation-size football still dwarfed his child’s hands. All told he spent 16 years on the gridiron, 16 years of using his body like a battering ram, 16 years of boom.

When he died two years ago, at 25, those 16 years left his brain brittle and deformed, pockmarked with the clumps of protein characteristic of chronic traumatic encephalopathy. The disease, researchers reported Monday, was the worst they had ever seen in someone so young.

“It was quite widespread,” Ann McKee, co-author on a study of Keck’s brain in the journal JAMA Neurology, told NBC News. (The study does not name Keck, but he has been identified by news outlets.)

“I have to say, I was blown away,” McKee told the Kansas City Star in November, before the study was published. “This case still stands out to me personally. It’s a reason we do this work. A young man, in the prime of his life, newly married, had everything to look forward to. Yet, this disease is destroying his brain.”

CTE can only be diagnosed after death, so it is rare when scientists are able to study its effects in someone as young as Keck. He is the youngest person to be definitively diagnosed with the disease, and McKee said its presence offers insight into how concussions affect young athletes, even ones who never play professionally.

“This case, as well as many others, shows us that contact sports athletes at the amateur level are also at risk for the disease,” she told Reuters by email. She and her colleagues suggest that young athletes who suffer repeated head trauma and show the symptoms of CTE be monitored for the disease.

Hospital-bound with an unrelated heart infection two years ago, just before he died of cardiac arrest, Keck made his wife pledge to donate his brain to McKee’s CTE lab at Boston University. The degenerative disease, which is often associated with football but also has been found in people who suffered an accident or played other contact sports, is the product of repeated brain trauma and successive concussions. And even though it wasn’t diagnosed, Keck and his wife were certain that he had it.

“It’s what he wanted to prove to everybody,” Cassandra Keck told the Associated Press when Keck’s illness was diagnosed last year. “He was really suffering, and nobody believed him.”

Keck was resilient, a survivor. His parents were addicted to drugs, according to the Kansas City Star, and his grandmother told the paper that they did drugs while the kids were in the house. Once, they burned a hole through a table and into the floor. Ultimately the grandmother, Charlotte Keck, took full custody of Michael. He grew up big-hearted and without resentment; once, when he heard his mother had been arrested, he told Charlotte Keck, “I hope she gets the help she needs.”

He also grew up tough. Keck began playing tackle football at age 6, and before long he was an expert at playing through pain.

“He told me that when he was 8, he would be sitting on the bench holding his head and the coach would be rubbing it because it hurt so bad,” Cassandra Keck told NBC News. Keck was taught that”if you’re hurt, you just get out there and play — don’t be a wuss,” she added.

So he did. He went out and played.

By high school, many thought the tall, broad blonde could make a career out of football. Many in Harrisonville, Mo. — a town of about 10,000 — expected Keck to go to a big-name college, somewhere they could see him play on TV.

But the young man shied away from the limelight, friends told the Kansas City Star. Although he was offered scholarships at the universities of Alabama, Michigan and Southern California, he opted for the University of Missouri — the smallest program he could play for without letting people down, friends said.

During his freshman year, according to Reuters, Keck suffered a blow to the head that left him unconscious. It was not his first such injury — he had his first concussion when he was only 8 — but it was the worst. Keck developed painful headaches, neck pain and blurry vision. He was distracted by a constant ringing in his ears. His sleep suffered, and his mood darkened. He became anxious and irritable, then violent.

Keck switched to Missouri State after his sophomore year, but wound up playing just one game for the school. Early in the season, he took a massive hit to the side of his head, according to the Kansas City Star. It was the worst collision he’d endured, and it ended his career.

His wife, Cassandra, was pregnant, and his mind was already beginning to fray. Keck walked into coach Terry Allen’s office and told him he’d had enough. He needed to stay healthy for his kid.

“We both had tears in my office,” Allen told the Star. “He was a great kid. Loved to play. I loved that kid. That’s a hard deal.”

But Keck’s condition continued to deteriorate.

“When he stopped playing, he became the bad seed,” Cassandra Keck told the Associated Press in 2014. “If they drank together, there ended up being holes in the wall. The next few years, people stopped coming around. People didn’t want to be around him anymore.”

Debilitating headaches kept him from reading, which kept him from graduating. Keck went to work until he couldn’t even do that. He smoked marijuana daily to relieve his headaches and anxiety, according to the Los Angeles Times, and he had become verbally and physically abusive toward his wife.

Meanwhile, he underwent a series of neurological tests that showed he suffered from memory problems and speech and language impairment. His vision blurred. Words scrambled in his head. He recognized less and less of the world around him. He barely recognized himself.

About the same time, former San Diego Chargers linebacker Junior Seau committed suicide. An autopsy would later reveal that Seau had CTE. His family filed a wrongful death lawsuit against the NFL, alleging that the linebacker’s suicide was a result of his brain disease.

Cassandra Keck said she immediately recognized Seau’s story.

It “really opened my eyes,” she told the Associated Press, “because that’s what Michael was going through.”

Keck died from an unrelated heart condition in 2013. His son, Justin, was 3-years-old.

An autopsy of Keck’s brain revealed a diagnosis of CTE last year. The case study published Monday details the pathology of his illness and discusses its ramifications for young athletes. Keck’s primary diagnosis is post-concussive syndrome, with secondary diagnoses of depression and CTE.

Researchers at Boston University found that Keck’s CTE was as severe as what had been found in the brain of 43-year-old Seau, even though Keck was almost two decades younger than Seau.

McKee told NBC News that the damage to Keck’s brain was probably so severe because he took up tackle football at such a young age.

“He started playing when he was quite young,” she said. “So he had 16 years of football behind him. That’s a lengthy exposure. Brain injury is cumulative.”

Since Keck’s case is the first to include a neuropsychological test profile — the results of all those tests he took in the short time between quitting football and his death — researchers say it can offer insight into what CTE looks like while the patient is still alive.

Columbia University neurologist James Noble, who wrote an editorial to accompany the article, told Stat News that he hopes this case will bring attention to amateur athletes who may be at risk for a degenerative disease.

“Those are the people we probably need to know a lot more about,” Noble said. “But we don’t.”