A gunman who killed four people at a Manhattan office building before killing himself claimed in a note to have a brain disease linked to contact sports and was trying to target the National Football League’s headquarters but took the wrong elevator, officials said Wednesday.

Investigators believe Shane Tamura, a Las Vegas casino worker, was trying to get up to the NFL offices after shooting several people in the skyscraper’s lobby on Tuesay, but entered the wrong elevator bank, Mayor Eric Adams said in interviews.

Four people were killed, including an off-duty New York City police officer, Didarul Islam.

The ambulance carrying the body of Didarul Islam exits NewYork-Presbyterian/Weill Cornell Medical Hospital.

The gunman blamed the NFL

Tamura, who played high school football in California nearly two decades ago but never in the NFL, had a history of mental illness, police said. In a three-page handwritten note found in his wallet, he claimed he suffered from chronic traumatic encephalopathy and 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, but it can only be diagnosed after someone has died.

In the note, Tamura repeatedly said he was sorry and asked that his brain be studied for CTE, according to the police department. It also referenced 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.

Members of the NYPD Crime Unit examine a door with bullet holes at the scene of the previous day's deadly shooting.

NFL Commissioner Roger Goodell called the shooting “an unspeakable act of violence in our building,” saying he was deeply grateful to the law enforcement officers who responded and the officer who gave his life to protect others.

Goodell said in a memo to staff that a league employee was seriously injured in the attack and was hospitalised in stable condition.

The shooting happened along Park Avenue, one the nation’s most recognized streets, and just blocks from Grand Central Terminal and Rockefeller Centre. It’s also less than a 15-minute walk from where UnitedHealthcare chief executive Brian Thompson was shot and killed last December by a man who prosecutors say was angry over corporate greed.

US President Donald Trump said Tuesday that he knows that area of Manhattan well. “My heart is with the families of the four people who were killed, including the NYPD Officer, who made the ultimate sacrifice,” Trump posted on social media.

Video showed the gunman stroll into the building

Investigators found that Tamura, who worked security at the Horseshoe Las Vegas, drove across the country the past few days and into New York City just before the shooting, Police Commissioner Jessica Tisch said.

This undated image provided by Las Vegas Dept. of Motor Vehicle shows Shane Tamura.

Surveillance video showed the gunman getting out of a BMW Tuesday and strolling across a plaza in a button-down shirt and jacket with a rifle at his side before he entered the building, which also has offices for the investment firm Blackstone and other companies. It was closed Wednesday except to investigators.

Once inside, he sprayed the lobby with gunfire, killing Islam, who was off-police duty and working a corporate security detail, and hitting a woman who tried to take cover, Tisch said. He then made his way to the elevator bank, shooting a guard at a security desk and another man in the lobby, she said.

“He appeared to have first walked past the officer and then he turned to his right, and saw him and discharged several rounds,” Adams said in a TV interview.

Tamura took an elevator to the 33rd-floor offices of the company that owns the building, Rudin Management, and shot and killed one person there before fatally shooting himself, the commissioner said.

Blackstone confirmed that one of its employees, real estate executive Wesley LePatner, was among those killed. Security officer Aland Etienne also died, according to a local labour union.

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The off-duty officer was from Bangladesh

Islam, 36, had served as a police officer in New York City for three-and-a-half years and was an immigrant from Bangladesh, Tisch said at a news conference.

His body was draped in the New York Police Department flag as it was moved from the hospital to an ambulance, with fellow officers standing at attention.

“He was doing the job that we asked him to do. He put himself in harm’s way. He made the ultimate sacrifice,” Tisch said. “He died as he lived: a hero.”