The gunman who believed he had CTE when he took the lives of four individuals and his own at a Manhattan office in July was confirmed to have the degenerative brain disease, according to a medical examiner on Friday.

Shane Devon Tamura, 27, who participated in high school football, was found to have “clear diagnostic evidence” of an early stage of CTE, or chronic traumatic encephalopathy, stated the medical examiner in New York City.

This brain condition, attributed to repeated head trauma, is widely recognized in the U.S. among NFL athletes. However, researchers acknowledge that Tamura is not an isolated case of a young athlete developing CTE, despite ceasing to play sports post high school or college.

“We are aware of numerous instances like this. High school football participants are susceptible to CTE,” commented Chris Nowinski, co-founder and CEO of the Concussion Legacy Foundation nonprofit.

Nowinski emphasized that the discovery of Tamura’s condition should alert football coaches and sports leaders to the necessity of enforcing stricter protocols in America’s favorite sport.

“We don’t need any more research to convince high school coaches to change the rules of the game,” he said. “There needs to be a will to do it.”

During Tamura’s autopsy, the medical examiner performed a brain examination following a note left at the site of the July 28 incident, where the attacker expressed frustration that his mental issues might be linked to his football playing and requested a CTE brain examination.

Authorities said Tamura was trying to target the headquarters of the NFL, which is in the Park Avenue building where the shooting took place.

Tamura played high school football in California about a decade ago and frequently took hits from other players in his positions as running back and defensive back, his former teammates and an ex-coach previously told NBC News.

Nowinski said news coverage of CTE cases has largely centered around high-profile NFL players, which he said creates the false perception that the issue only affects professional athletes. But he said experts who study brain injuries know that high school athletes are also at risk.

It’s a pressing concern, said Nowinski, who played college football for Harvard University and sustained a career-ending concussion in 2003 as a WWE professional wrestler.

Nowinski said he hopes Tamura’s case leads high school football associations to educate coaches and players about CTE and set stricter limits on how many times, and how hard, athletes can be hit in the head.

“We’d be insane not to enact efforts to prevent CTE,” he said.

An estimated over 300,000 high school athletes in the country suffer a concussion each year, the National Federation of State High School Association has previously said. In 2019, it recommended limiting full contact during practice sessions and limiting the total number of quarters or periods played per week in sports like football and ice hockey.

The NFL, which first acknowledged the link between CTE and football in 2016, did not immediately comment.

Former NFL player Aaron Hernandez had one of the worst cases of CTE for his age that researchers had seen, the Boston center said in 2017. The 27-year-old died by suicide that year after being convicted of murder.

Another ex-NFL player, Phillip ams, who fatally shot six people before taking his own life in 2021, was also found to have CTE.

Most recently, when former Pro Bowl running back Rudi Johnson died by apparent suicide this month, his agent called for greater CTE research and prevention, although it’s unclear if the three-time 1,000-yard rusher had it.

CTE can only be diagnosed after death since there is no imaging tool that can detect brain injuries like CTE while a person is alive, according to Dr. Ross Zafonte, a brain injury specialist and the executive vice dean at the University of Missouri’s School of Medicine.

That has made it difficult to study how CTE affects behavior, he said.

While Boston University’s CTE Center said brain degeneration is associated with common symptoms of CTE, including impulse control problems, aggression, depression and suicidality, Zafonte and two other experts who study brain injuries said CTE has not been definitively linked to behavioral changes.

Authorities said Tamura had a history of mental health issues and encounters with law enforcement. In 2022, he threatened to kill himself while suffering from depression, chronic migraines and insomnia, according to a phone call his mother made to police.

Zafonte said the latest high-profile case study might help researchers further understand the role of impulsivity. But, he cautioned, many factors affect behavior and researchers do not yet know the full depth of Tamura’s mental health history.

“We have to remember this is an evolving and changing piece of science,” he said.

If you or someone you know is in crisis, call or text 988, or go to 988lifeline.org, to reach the Suicide & Crisis Lifeline. You can also call the network, previously known as the National Suicide Prevention Lifeline, at 800-273-8255, or visit SpeakingOfSuicide.com/resources.