By James Oliphant

(Reuters) -The man who killed four people at a midtown Manhattan office tower housing the NFL's head office suffered from a traumatic brain injury commonly associated with football players, the New York City Medical Examiner’s Office said on Friday.

Shane Tamura, 27, a Las Vegas casino security officer and former high-school football player, had left a “suicide note” blaming the National Football League for his degenerative brain condition.

“Following a thorough assessment and extensive analysis by our neuropathology experts, OCME has found unambiguous diagnostic evidence of Chronic Traumatic Encephalopathy, also known as CTE, in the brain tissue of the decedent,” the office of the Chief Medical Examiner said in a statement. “The findings correspond with the classification of low-stage CTE, according to current consensus criteria.”

In the deadliest attack in New York City in a quarter century, Tamura killed two security guards, one of them a city police officer on security detail, as well as a real estate executive and a business management associate, before taking his own life on the 33rd floor of the Park Avenue skyscraper.

Even though the gunman failed to reach the league's offices, an employee of the NFL was gravely wounded by Tamura in the lobby of the building.

Tamura was never an NFL player, but he did play football during high school in California, according to school sports databases.

"Study my brain. I'm sorry," Tamura wrote in the note, according to police.

A posthumous brain autopsy is the only way to diagnose CTE.

After denying a connection between football and brain damage for years, the NFL publicly admitted in 2016 a link between head trauma and CTE.

The NFL has paid more than $1.5 billion to thousands of retired players after the deaths of several high-profile players. It has made changes to the sport to mitigate the risk of concussions.

(Reporting by James Oliphant in Washington; Editing by Frank McGurty and Lisa Shumaker)