The crazed gunman who killed an NYPD officer and three other people in a Midtown skyscraper on Monday evening was carrying a note in his pocket that expressed grievances with the NFL and claimed he suffered from CTE – a brain injury linked to head trauma, sources told The Post.

Shane Tamura, 27, cited the NFL in the writings, which were found after he fatally shot himself in the chest on the 33rd floor of 345 Park Ave. – a swanky skyscraper that houses the football league’s headquarters.

The mentally ill shooter wanted to shoot up the NFL HQ — but “mistakenly went up the wrong elevator banks” and ended up on a higher floor, Mayor Eric Adams confirmed Tuesday.

In the ramblings, which was several pages long, Tamura blamed football for his apparent struggle with the neurodegenerative disease chronic traumatic encephalopathy (CTE) and pleaded for his brain to be studied in the wake of the massacre, law enforcement sources said. 

He wrote addressing Terry Long, the former Pittsburgh Steelers player who was diagnosed with CTE after downing antifreeze to kill himself 20 years ago.

“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,” he wrote. “They failed us.”

The NFL has offices on floors five through eight of the Park Avenue high-rise, and one of its employees was among those shot, though not fatally..

Mayor Adams confirmed early Tuesday that Tamura’s note “alluded to having CTE from playing NFL” — even though “he never played in the NFL.”

“It appears as tho he was going after the employees of the NFL,” the mayor confirmed on Fox 5.

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

Shortly after the shooting, NYPD Commissioner Jessica Tisch directly called executives at Blackstone – another notable tenant in the skyscraper – to inform them that they weren’t the intended targets, sources said.

Tamura, who lived in Las Vegas and had a known mental health history there, initially opened fire in the building’s lobby before taking the elevator up to the 33rd floor where he turned the gun on himself.

He also shot a fifth victim in the lobby of the building who survived the attack, police said.

The surviving victim appears to an NFL employee, as the league’s commissioner, Roger Goodell, told staffers in a letter that one of their own had been “seriously injured” in the shooting.

Meanwhile, NYPD Commissioner Jessica Tisch had earlier revealed the maniac gunman, who lived in Las Vegas, had a known mental health history.

Tamura, who worked a security guard at a Las Vegas casino, never played professional football.

But he did play in high school in California, when he was a described as a standout football player.

“It looked like the sky was the limit, and then it wasn’t anymore,” former classmate Caleb Clarke told NBC News. Ahead of his senior year, his coach told the Los Angeles Times that he expected “big things” from the running back and other star players on the team.

NEW YORK — A man stalked through a Manhattan office tower firing a rifle Monday, killing four people, including a New York City police officer, and wounding a fifth before taking his own life, officials said.

The shooting took place at a skyscraper that is home to the headquarters of both the NFL and Blackstone, one of the world’s largest investment firms, as well as other tenants.

The gunman, identified by authorities as Shane Tamura of Las Vegas, had a ‘documented mental health history,’ according to Police Commissioner Jessica Tisch, but his motive was still unknown.

“We are working to understand why he targeted this particular location,” Tisch said.

The rampage happened at the end of the workday in the same part of Manhattan where the chief executive of UnitedHealthcare was gunned down outside a hotel late last year.

Surveillance video showed the man exiting a double-parked BMW just before 6:30 p.m. carrying an M4 rifle, then marching across a public plaza into the building. Then, he started firing, Tisch said, killing a police officer working a corporate security detail and then hitting a woman who tried to take cover as he sprayed the lobby with gunfire.

The man then made his way to the elevator bank and shot a guard at a security desk and shot another man in the lobby, the commissioner said.

The man took the elevator to the 33rd floor offices of the company that owned the building, Rudin Management, and shot and killed one person on that floor. The man then shot himself, the commissioner said. The building, 345 Park Avenue, also holds offices of the financial services firm KPMG.

The officer killed was Didarul Islam, 36, an immigrant from Bangladesh who had served as a police officer in New York City for 3 1/2 years, Tisch said at a news conference.

“He was doing the job that we asked him to do. He put himself in harm’s way. He made the ultimate sacrifice,” Tisch said. “He died as he lived. A hero.” 

One man was seriously wounded and remains in critical condition, Mayor Eric Adams said. Four others got minor injuries attempting to flee.

Adams said officials are still “unraveling” what took place.

Officers found a rifle case, a revolver, magazines and ammunition in Tamura’s car, Tisch said. They also found medication that belonged to Tamura, she said.

Rudin is one of the largest privately owned real estate companies in New York City. The company dates back to 1925 and is still managed by members of the Rudin family.

Tisch said there were no indications so far that Tamura had prior connections to the real estate industry or to the city. 

No one answered the door at the address listed for Tamura in Las Vegas. 

Islam, the slain officer, leaves behind two young boys, and his wife is pregnant with their third child, Tisch said.

Witnesses heard ‘rapid fire’

Local TV footage showed lines of people evacuating the office building with their hands above their heads in the hours after the killings.

Nekeisha Lewis was eating dinner with friends on the plaza when she heard gunfire.

“It felt like it was a quick two shots and then it was rapid fire,” she told The Associated Press. 

Windows shattered and a man ran from the building saying, “Help, help. I’m shot.” Lewis said.

Jessica Chen told ABC News she was watching a presentation with dozens of other people on the second floor when she “heard multiple shots go off in quick succession from the first floor.”

She and others ran into a conference room and barricaded tables against the door.

“We were honestly really, really scared,” she said, adding that she texted her parents to tell them that she loves them.

Some finance workers at an office building down the block were picking up dinner at a corner eatery when they heard a loud noise and saw people running. 

“It was like a crowd panic,” said Anna Smith, who joined the workers pouring back into the finance office building. They remained there for about two hours before being told they could leave.

Tisch says she believes two officers were working in different parts of the building as part of a program where companies can hire NYPD officers to provide security.

The building where the shooting happened is in a busy area of midtown, located a short walk north from Grand Central Terminal and about a block east of St. Patrick’s Cathedral.

Through late July, New York City is on pace this year to possibly have its fewest homicides and fewest people hurt by gunfire in decades. But the city’s corporate community has been on edge since last December, when UnitedHealthcare CEO Brian Thompson was killed outside a hotel hosting a conference.

The man charged in that killing, Luigi Mangione, is awaiting trial. Prosecutors accuse him of killing Thompson because he was angry at perceived corporate greed, particularly in the health insurance business. He has pleaded not guilty.