The man who killed four people inside a Manhattan office tower this summer was suffering from a degenerative brain disease, a city medical examiner said on Friday, confirming the gunman's own self-diagnosis.

Shane Tamura, 27, had “unambiguous diagnostic evidence” of low-stage chronic traumatic encephalopathy, commonly known as CTE, according to the New York City medical examiner.

Tamura, a Las Vegas casino worker who had played football in high school, carried out the mass shooting on July 28, spraying bullets into the lobby of a Manhattan office building housing the headquarters of the NFL, which he accused of hiding evidence of the brain injury.

Among the dead were a police officer, a security guard and two people who worked at companies in the building. An NFL employee was badly wounded but survived.

In a three-page note found in his wallet, Tamura said he had chronic traumatic encephalopathy – diagnosable only after death – and implored those who found him: “Study my brain.”

Among his grievances against the NFL was a claim that the league put its profits ahead of player safety by concealing the harm CTE, and football, can cause. (AP)