NEW YORK (AP) — Investigators are piecing together more details about how a former high school football player who blamed the game for his mental health problems carried out a deadly attack on an office building that is home to the NFL.

Shane Tamura killed four people on Monday before killing himself, spraying the skyscraper’s lobby with bullets and then continuing his rampage on the 33rd floor, authorities said.

Inside his wallet, a handwritten note claimed he had chronic traumatic encephalopathy, known at CTE, and accused the NFL of hiding the dangers of brain injuries linked to contact sports, investigators said.

Detectives were still working to unravel more details about the 27-year-old’s background and motivations.

They planned to question a man who supplied gun parts for the AR-15-style rifle used in the attack, including the weapon’s lower receiver, Police Commissioner Jessica Tisch said in a video statement.

Among the dead were a police officer, a security guard and two people who worked at companies in the building. An NFL employee was badly wounded but survived.

Tamura, a Las Vegas casino security worker, had intended to target the NFL’s headquarters in the building but took the wrong elevator, officials said.

It’s unclear whether he showed symptoms of CTE, which can be diagnosed only by examining a brain after death.

Tamura, who played high school football in California a decade ago but never played in the NFL, had a history of mental illness, police said without giving details. In the three-page note found on his body, he accused the NFL of concealing the dangers to players’ brains for profit. The degenerative brain disease has been linked to concussions and other repeated head trauma common in contact sports such as football.

At a Tuesday night vigil for those killed in the shooting, Muslim, Sikh, Buddhist, Hindu, Christian and other faith leaders delivered prayers at a park about a dozen blocks from where the shooting took place.

Mayor Eric Adams and Gov. Kathy Hochul spoke of the need for stronger gun laws.

“We cannot respond to senseless gun laws through vigils,” Adams said.

NFL boss calls shooting ‘unspeakable’

Tamura’s note repeatedly said he was sorry and asked that his brain be studied for CTE.

The NFL long denied the link between football and CTE, but it acknowledged the connection in 2016 testimony before Congress and has paid more than $1.4 billion to retired players to settle concussion-related claims.

NFL Commissioner Roger Goodell, who works out of the offices, called the shooting “an unspeakable act of violence.”

The shooting happened at a skyscraper on Park Avenue, one of the nation’s most recognized streets, just blocks from Grand Central Terminal and Rockefeller Center. It is less than a 15-minute walk from where UnitedHealthcare CEO Brian Thompson was shot and killed last December by a man who prosecutors say was angry over what he saw as corporate greed.

Video shows the gunman stroll into the building

Tamura drove across the country in the days before the attack and into New York City, Tisch said. Surveillance video showed him exit his BMW outside the building at about 6:30 p.m. Monday wearing a button-down shirt and jacket with the rifle at his side.

Once inside the lobby, he opened fire and killed Islam and Wesley LePatner, a real estate executive at the investment firm Blackstone, which occupies much of the building. Tamura then made his way toward the elevator bank, shooting the NFL employee and an unarmed security guard, Aland Etienne, who helped control access to the upper floors.

Tamura waited for the next elevator to arrive in the lobby, let a woman walk safely out of the elevator, then rode it up to the 33rd-floor offices of the company that owns the building, Rudin Management. He killed a worker for that company before killing himself, officials said.

Friends and family mourn killed officer

Officer Didarul Islam, 36, who was guarding the building on a paid security job when he was killed, had served as a police officer in New York City for over three years. He was an immigrant from Bangladesh and was working a department-approved building security job when he was shot.

Islam leaves a pregnant wife and two children. Friends and family stopped by their Bronx home on Tuesday to drop off food and pay their respects.

“He was a very friendly guy and a hardworking guy,” said Tanjim Talukdar, who knew him best from Friday prayers. “Whenever I see him or he sees me, he says, ’How are you, my brother?’”

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Collins reported from Hartford, Connecticut, and Seewer from Toledo, Ohio. Associated Press reporters Michael Balsamo, Philip Marcelo and Julie Walker in New York; Maryclaire Dale in Philadelphia, Rob Maaddi in Tampa, Florida; Mike Catalini in Trenton, New Jersey; and Eric Tucker in Washington contributed to this report.