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Tamura, 27, with bullets then shot another person in a 33rd-floor office on Monday before he killed himself, authorities said. Among 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.

The attacker's grievances with the NFL emerged as police worked to piece together his background and motivations, and as loved ones began to mourn .

It's unclear whether Tamura 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 has been linked to concussions and other repeated head trauma common in contact sports such as 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.

NFL boss calls shooting “unspeakable�

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.

, 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 was shot and killed last December by a man who 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.�

Video shows the gunman stroll into the building

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 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

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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This story was updated to correct that Tamura played high school football about a decade ago, not nearly two decades ago.

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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.