The crazed gunman behind the deadly Manhattan shooting left behind a note referencing former NFL star Terry Long — the Pittsburgh Steelers player who was diagnosed with CTE after downing antifreeze to kill himself, sources said.

Shane Tamura, 27, killed an NYPD officer and three other people on Monday evening when he stormed the Midtown skyscraper that’s home to the football league’s headquarters.

In the wake of the bloodshed, cops discovered a rambling note in which Tamura mentioned the ex-NFL star and claimed that he, too, suffered from CTE — a brain injury linked to head trauma.

“Terry Long football gave me CTE and it caused me to drink a gallon of antifreeze,” the note stated, according to sources.

“You can’t go against the NFL, they’ll squash you.”

“Please study brain for CTE. I’m sorry. The league knowingly concealed the dangers to our brains to maximize profits. They failed us.”

Long, who was an offensive lineman for the Pittsburgh Steelers in the late 1980s, was diagnosed with CTE after he killed himself in 2005 by swallowing antifreeze.

Mayor Eric Adams confirmed Tuesday that the mentally ill shooter appeared to have been targeting the NFL’s headquarters, which has offices in the Park Avenue high-rise.

Follow The Post’s live blog for the latest coverage of the 345 Park Ave. shooting

“It appears as though he was going after the employees of the NFL,” Adams said during an appearance on Fox 5.

“We’re still going through the suicide note to zero in on the exact reason, but at this time it appears as if it’s something attached to his belief he experienced CTE from the NFL.”

Despite suggesting he suffered from CTE from playing in the NFL, Tamura never played professional football.

He played high school football in California, where he was described as a standout player.

In the wake of Monday’s massacre, NYPD Commissioner Jessica Tisch said the maniac gunman, who lived in Las Vegas, had a known mental health history.

How the shooting unfolded

• Reports of the shooting at 345 Park Ave. start coming in around 6:28 p.m.
• Shane Tamura, 27, is seen getting out of a black BMW between 51st and 52nd streets with an M4 rifle.
• He enters the lobby and turns right, where he shoots police officer Didarul Islam, 36, dead.
• Tamura guns down a woman cowering behind a pillar in the lobby, sprays more bullets and walks toward the elevator bank — where he shoots dead a security guard crouching at his desk.
• One more man reports being shot and injured in the lobby. He was in critical but stable condition.
• The gunman allows a woman to walk out of the elevators unharmed before heading up to the 33rd floor, where building owner Rudin Properties’ offices are located, “and begins to walk the floor, firing as he traveled.”
• One woman is shot and killed on that floor before Tamura shoots himself in the chest.
• It’s unclear how long the mayhem lasted. Police Commissioner Jessica Tisch posted on X at 7:52 p.m.: “The scene has been contained and the lone shooter has been neutralized.”

He stormed the Midtown office building at 345 Park Ave. and initially opened fire in the lobby — killing several people, cops said.

He then “mistakenly went up the wrong elevator banks” and ended up on a higher floor than the NFL’s offices, which are spread across levels 5 through 8, the mayor said.

Tamura was found dead on the 33^rd floor after turning the gun on himself, according to cops.