The crazed gunman behind the fatal rampage in a Midtown skyscraper was carrying a suicide note revealing he targeted the NFL’s headquarters — because he believed himself to suffer a football-related brain injury, The Post has learned.

The three-page missive found folded in shooter Shane Tamura’s wallet – after he shot and killed an NYPD officer and three other innocent victims and then himself Monday – railed against the NFL, law-enforcement sources said. 

Tamura’s scrawled ramblings blamed football for his perceived struggle with the disease chronic traumatic encephalopathy (CTE) — despite not playing the sport past high school.

“Please study brain for CTE. I’m sorry. The league knowingly concealed the dangers to our brains to maximize profits,” the note read.

“They failed us.”

CTE is a degenerative brain disease that is caused by concussions and repeated blows to the head. It has been found in hundreds of former NFL players.

Symptoms include aggression and other behavioral problems, memory loss and, in the late stages, dementia. It can only be diagnosed by dissecting the patient’s brain after death.

The city’s medical examiner will determine whether the 27-year-old ex-football player’s brain will be studied for CTE, Mayor Eric Adams told CNN on Tuesday.

Authorities, as part of the sprawling probe into Tamura’s senseless mass shooting inside 345 Park Avenue, were looking into an exact motive for the bloodshed.

Detectives have already discovered a history of mental illness — including self-reporting that he was suicidal.

In another note, found inside Tamura’s Las Vegas, Nevada apartment Tuesday, said something like:  “When I look into you and dad’s eyes, I see complete disappointment,” referring to the gunman’s parents, according to sources.

Investigators believe Tamura took a cross-country trek in his BMW, with the flashy car being spotted in Colorado on July 26, in Nebraska and Iowa a day later and New Jersey on Monday, sources said. 

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.”

The car was ultimately found double-parked between 51st and 52nd streets, where surveillance footage spotted Tamura toting his homemade AR-15 assault rifle and striding into the Park Avenue highrise.

Cops found a Colt Python .357-caliber handgun legally purchased by Tamura inside his car, along with a rifle case, ammunition and the migraine medication Sumatriptan.

The psycho stormed the building’s lobby at around 6:30 p.m., killing three people – NYPD Officer Didarul Islam, Blackstone exec Wesley LePatner and security guard Aland Etienne – and wounded a fourth, who happened to work for the NFL, officials and sources said. 

He then tried to reach the NFL’s offices on levels 5 through 8, but “mistakenly went up the wrong elevator banks” and ended up on the 33rd floor housing Rudin Management, Adams confirmed. 

Rudin employee Julia Hyman was Tamura’s last victim before he fatally fired a bullet into his chest, avoiding his head in an apparent bid to preserve his brain for research, Adams told PIX11. 

“But at this time, we do not know of any communication that he had with the NFL at all,” Adams said.

The suicide note in Tamura’s wallet specifically referred to 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.”

Tamura also scribbled “Frontline Documentary” on one of his suicide note’s pages – an apparent reference to the “League of Denial” doc that probed the links between the NFL and the brain injury linked to head trauma.

He also mentioned the ESPN reporters who co-wrote the “League of Denial” book and named several doctors, including Dr. Ann McKee, who is the Chief of Neuropathology at BU Alzheimer’s Disease Research Center, Dr. Christopher Nowinski, the co-founder of Boston University’s CTE Center, and Bennet Omalu.

Omalu was played by Will Smith in Concussion, a 2015 film about the forensic pathologist going against the NFL on the degenerative brain disease.

Tamura, who worked in security at Horseshoe Las Vegas casino, never played professional football.

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

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But Tamaru’s coach said he doesn’t believe his ex-gridiron star ever suffered a head injury, at least during his senior year.

“I only remember an ankle,” former Granada Hills Charter School football coach Walter Roby said.

“I remember it might have kept him out of one game, but something of that nature. I can’t remember if it was left or right.”

A former Granada Hills Charter teammate who played with him also doesn’t remember him suffering a head injury, emphasizing their division wasn’t as intense as other parts of California.

“I think Tamura was healthy,” said the 28-year-old who did not want to be identified in an interview. “I don’t remember any injuries.”

After high school, Tamura lived in Las Vegas, where he had a known mental health history.

A group of NYPD detectives investigating the shooting is in Las Vegas, searching his home and questioning his casino boss, who originally bought the lower receiver for the AR-15-style weapon used in the rampage, sources said.

Tamura applied for a gun permit in April 2022 and was allowed to hold onto it even after he told Las Vegas officials afterward that he was suicidal, sources said.

He had a valid concealed carry weapons permit in Nevada.

Authorities logged another mental health contact with Tamura in 2024, according to sources.

In June, police received a tip that Tamura purchased large amounts of ammunition and an after-market trigger at a gun show, sources said.

NYPD Commissioner Jessica Tisch said Tamura appears to have assembled the weapon himself after obtaining the central piece – the lower receiver.

Tamura had been scheduled to work an overnight shift at his casino job Sunday, but he never showed up, police said. 

– Additional reporting by Emily Crane, Tina Moore and Alex Oliveira

If you are struggling with suicidal thoughts or are experiencing a mental health crisis and live in New York City, you can call 1-888-NYC-WELL for free and confidential crisis counseling.

If you live outside the five boroughs, you can dial the 24/7 National Suicide Prevention hotline at 1-800-273-8255 or go to SuicidePreventionLifeline.org.