The gunman who killed four people in a Manhattan office building had chronic traumatic encephalopathy (CTE), a degenerative brain disease often linked to high-contact sports like American football, according to the New York City medical examiner's office.

In July, Shane Tamura, 27, drove from Las Vegas to New York City with an assault-style rifle, shooting four in a Park Avenue skyscraper before killing himself.

Police believe he was targeting the offices of the National Football League (NFL), which were inside the building.

In notes left at the scene, Tamura reportedly blamed the NFL for concealing the effects of CTE and asked for his brain be studied.

CTE can only be diagnosed post-mortem.

New York City's chief medical examiner confirmed that the shooter's autopsy revealed he had low-stage CTE with an unambiguous diagnosis. The disease is caused by repeated blows to the head and has been found in the brains of dozens of former NFL players. Science surrounding it is evolving, with associations to symptoms including memory loss, depression, and progressive dementia.

The physical and mental manifestations of CTE remain under study, the medical examiner noted.

Tamura, a former high school football player who did not play in the NFL, mistakenly targeted a part of the building where the American football league headquarters was located while attempting to access a different floor.

His alleged suicide note included a plea for his brain to be studied and an apology: study my brain please and I'm sorry, according to a senior official who spoke to ABC News.

Authorities have stated that Tamura was known to have mental health issues.

Among those killed in the shooting were NYPD officer Didarul Islam, Wesley LePatner, an employee of finance giant Blackstone, and Julia Hyman, an employee at Rudin Management. NFL employee Craig Clementi was also seriously injured.

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