NEW YORK — Shane Tamura, the Park Avenue gunman who killed an NYPD officer and three others before taking his own life, left behind a suicide note saying he suffered from CTE, a brain injury often linked to playing football, and was targeting the NFL — but took the wrong elevator, officials said Tuesday.

Tamura, 27, who played competitive football when he was young, took issue with the NFL in the note. He never played for the NFL

“You can’t go against the NFL,” he wrote, police sources said. “They’ll squash you.”

The note, which is three pages long and which Tamura had on him as he carried out his shooting rampage Monday, says he wants his brain to be studied, police sources said.

One in three former NFL players, according to a study, believe they have CTE, chronic traumatic encephalopathy. The neuropathological condition is linked to repeated head trauma but can only be diagnosed through a postmortem exam.

“Please study brain for CTE. I’m sorry,” Tamura wrote in the note, the sources said. “The league knowingly concealed the dangers to our brains to maximize profits. They failed us.”

The NFL has offices on four lower floors of the building where the shooting occurred.

“We have reason to believe that he was focused on the NFL agency that was located in the building,” Mayor Adams told MSNBC Tuesday morning. Adams confirmed he left a suicide note that talked about CTE.

The shooting however, took place in the lobby and on the 33rd floor, home to Rudin Management, where police say he killed one woman before shooting himself in the chest.

“He mistakenly went up the wrong elevator bank,” Adams told PIX11 Tuesday, explaining how Tamura ended up on the wrong floor.

Mayor Adams told MSNBC that safeguards on the 33rd floor saved lives.

“On the floor itself, there were safe rooms, such as the bathrooms, where you had bulletproof doors and you were able to lock in the staffers inside,” he said. “Several individuals were able to lock themselves in the safe rooms and we believe it played a major role in not having a greater loss of life.”

Tamura lived in Las Vegas and had a history of mental illness, NYPD Commissioner Jessica Tisch said at press conference Monday night.

In his suicide note, he wrote about Terry Long, a former offensive lineman for the Pittsburgh Steelers, who at first was said to have died in 2006 of cerebral meningitis related to the CTE he was suffering from. A coroner later determined he committed suicide by drinking antifreeze.

“Football gave me CTE and it cause me to drink a gallon of antifreeze,” Tamura wrote.

Tamura drove cross-country and double-parked his black BMW when he arrived at 345 Park Ave., a 44-story skyscraper that takes up an entire block, from E. 51st to E. 52nd St.

Toting an M4, an assault-style rifle, he walked into the lobby of the building and opened fire, Tisch said, first killing NYPD Officer Didarul Islam, who was working his second job as a security officer at the building, Tisch said.

The gunman shot two other people in the lobby, killing one and badly wounding the other, before taking the elevator to the 33rd floor, Tisch said.

The critically wounded survivor is an NFL executive, police sources said. He has since stabilized, NFL Commissioner Roger Goodell wrote in a memo to staffers, according to ESPN.

“NFL staff are at the hospital and we are supporting his family,” Goodell wrote. “We believe that all of our employees are otherwise safe and accounted for.”

Goodell encouraged employees to work from home Tuesday as the investigation continues and said there would be increased security at the building in the weeks ahead.

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With Thomas Tracy

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