NEW YORK, United States (AFP) — A man who gunned down four people then killed himself in a Manhattan skyscraper may have targeted the American football league because he blamed it for brain injuries he claimed he suffered, Mayor Eric Adams said Tuesday.

Information that the killer carried in a note referring to the degenerative brain disease CTE offered a possible motive for the Monday shootings at offices used by the National Football League (NFL), among others.

A source confirmed that in a three-page handwritten note found in the gunman’s wallet, he wrote “Chronic traumatic encephalopathy (CTE). Study my brain please. I’m sorry.”

“Football gave me CTE and it cause me to drink a gallon of antifreeze. You can’t go against the NFL. They’ll squash you.”

The suspected shooter, identified as 27-year-old Shane Tamura, went on to ask that his brain be studied for CTE, and alleged that the league “knowingly concealed the dangers to our brains to maximize profits.”

Armed with a semi-automatic rifle, the attacker shot a police officer outside the tower on Park Avenue, then opened fire in the lobby before trying to access the NFL’s offices.

Tamura had never actually played for the top professional league, Adams said, though he was reportedly a star player in high school in California.

The bloodshed sparked a massive police response in the teeming center of the city — not far from where a man with a grievance against UnitedHealthcare gunned down the medical insurance company’s CEO last December.

New York Police Commissioner Jessica Tisch told a news conference that Tamura had a history of mental health issues.

President Donald Trump on Tuesday called the mass shooting a “senseless act of violence” carried out by a “lunatic.”

Deadly journey

New details emerged Tuesday about the gunman’s bloody attack and final journey.

Authorities said he had driven across the country from Nevada and stopped outside the skyscraper in a black BMW, carrying a rifle.

He killed a police officer immediately, then began “spraying the lobby” with bullets,” Tisch said.

A female bystander and a security guard were hit, Adams said, explaining that this prevented anyone pushing a panic button that would have stopped the elevators from working.

As the guard “attempted to hide himself behind the counter, he was killed as well, and the suspect then took the elevator upstairs,” Adams said.

“If he was able to get to the button, he could have froze the elevator.”

One of the victims shot was an NFL employee, who was “seriously injured” but stable in hospital, league commissioner Roger Goodell said in a statement to employees.

But while his target was determined to be the NFL offices, Tamura “took the wrong elevator,” Adams said, ending up on the 33rd floor, which houses the building’s management. He murdered one person there and then shot himself in the chest.