NEW YORK — The shooter who killed four individuals within a Manhattan office building attributed his mental health struggles to the National Football League. His aim was to attack its headquarters, but he mistakenly took the wrong elevator, officials reported on Tuesday.

Shane Tamura, a security worker from a Las Vegas casino, had a handwritten note in his wallet asserting that he suffered from chronic traumatic encephalopathy, or CTE, investigators revealed. He accused the league of masking the risks associated with brain injuries from contact sports.

Tamura, 27, unleashed gunfire in the lobby of the skyscraper before shooting another individual on the 33rd floor on Monday, eventually taking his own life, authorities stated. Among those killed were a police officer, a security guard, and two employees from companies within the building. An NFL staff member sustained serious injuries but survived.

As the police gathered details about Tamura’s past and motivations, the gunman’s issues with the NFL came to light, while families started to grieve their losses.

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

Details from Tamura’s life, including a history of mental health issues, emerged, though specifics were not provided, according to the police. A triad of notes found on him accused the NFL of profiting while concealing threats to players’ brain health. This brain disease is connected to concussions and recurring head injuries typical in contact sports like football.

Detectives 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.

She and members of the force also paid tribute to Officer Didarul Islam, who was guarding the building on a paid security job when he was killed. His flag-draped remains arrived late Tuesday afternoon at the Bronx mosque preparing for his funeral.

Tamura’s note repeatedly said he was sorry and asked that his brain be studied for CTE. He mentioned a PBS Frontline documentary about the disease and referred to former NFL player Terry Long, who was diagnosed with CTE, and the manner in which Long killed himself in 2005.

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,” saying he was deeply grateful to the law enforcement officers who responded.

Goodell said in a memo to staff that the injured NFL employee was hospitalized in stable condition.

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.

Monday’s attack drew a response from the White House, with President Donald Trump posting that his “heart is with the families of the four people who were killed” and that the officer “made the ultimate sacrifice.”

Tamura, who worked in the security department at the Horseshoe Las Vegas but failed to show up for his shift Sunday, drove across the country over the past few days and into New York City just before the attack, Tisch said.

Surveillance video showed Tamura 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.

Islam, 36, had served as a police officer in New York City for over three years and was an immigrant from Bangladesh, Tisch said. He was working a department-approved job, in his New York Police Department uniform, 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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