A gunman who shot dead four people in a Manhattan skyscraper was trying to target the National Football League headquarters, as authorities investigate a note found on his body claiming he wanted his brain studied after his death.

New York’s mayor Eric Adams said Las Vegas local Shane Devon Tamura, 27, was trying to enter the NFL offices but took the wrong lift on Monday, local time, when he shot dead four people, including an off-duty police officer, with an M4 assault rifle.

A note found on his body expressed a grievance with the NFL amid unconfirmed claims he was suffering from C.T.E — a degenerative brain disease — from repeated head traumas in contact sports such as American Football.

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“He seemed to have blamed the NFL,” the mayor said.

“The NFL headquarters was located in the building, and he mistakenly went up the wrong elevator bank.”

AP reports two people familiar with the matter said the note said his brain should be studied after he died.

Tamura, who Mr Adams said never played in the NFL, was found with a fatal self-inflicted gunshot wound inside the 44-storey building, which is also home to investment firm Blackstone.

The horrifying ordeal happened at 345 Park Avenue, building blocks away from St. Patrick’s Cathedral.

Mr Adams, in the wake of the shooting which also reportedly injured an NFL employee, said Tamura had a “documented” mental health history.

He described the attack on innocent people as an “act of evil”.

“No words can describe this act of evil, a man who takes a life from others who are innocent. No words can fill the void that has been left by this tragedy. Our hearts break for the families and friends of the victims,” he said.

The 27-year-old was captured on CCTV carrying the firearm by his waist-side on the street before he entered the Midtown building that houses Rudin Management, KPMG, Blackstone and the NFL headquarters.

It is reported Tamura barricaded himself into a room on the 33rd floor before turning the gun on himself.

The police officer killed in the attack has been identified as Didarul Islam, a 36-year-old immigrant from Bangladesh who had served with the NYPD for three and a half years.

The second victim has been identified as Blackstone executive Wesley LePatner, a senior managing director at the firm and a mother of two children.

Tamura arrived in New York hours before the attack, which was about 6.30pm local time on Monday.

He had been driving cross-country for days after leaving his home in Nevada.