The gunman who suspected he had CTE when he killed four people and himself in a Manhattan office building in July did in fact have the degenerative brain disease, a medical examiner said Friday.

Shane Devon Tamura , 27, who played high school football, had "unambiguous diagnostic evidence" of low-stage CTE, or chronic traumatic encephalopathy, the New York City medical examiner said.

The brain disease is caused by repeated head injuries and is most commonly associated in the U.S. with NFL players. But researchers say Tamura is far from the first young athlete to develop CTE despite not having played sports after high school or college.

"We know that we have a lot of cases like this. We know high school football players can get CTE," said Chris Nowinski, co-founder and CEO o

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