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 to be studied.
CTE can only be diagnosed post-mortem.
New York City's chief medical examiner confirmed that Tamura's autopsy revealed low-stage CTE and the diagnosis was unambiguous.
The disease, caused by repeated blows to the head, has been found in the brains of dozens of former NFL players and is associated with symptoms including memory loss, depression, and progressive dementia.
The physical and mental manifestations of CTE remain under study, they said.
Tamura, a former high school football player, aimed for the American football league's headquarters but mistakenly entered a different part of the building. His alleged suicide note requested, study my brain please, and included an apology.
Police reported that Tamura had known mental health issues.
Among the victims of the shooting were NYPD officer Didarul Islam, Wesley LePatner of finance giant Blackstone, and Julia Hyman from Rudin Management, with NFL employee Craig Clementi seriously injured.