This image from surveillance video obtained by The Associated Press shows Shane Tamura outside a Manhattan office building on Monday in New York. (AP Photo)

NEW YORK (AP) — A gunman who killed four people inside a Manhattan office tower blamed his mental health problems on the National Football League and intended to target the league’s headquarters upstairs but took the wrong elevator, officials said Tuesday.

Investigators said Shane Tamura, a Las Vegas casino worker, was carrying a handwritten note in his wallet that claimed he suffered from chronic traumatic encephalopathy, known at CTE, and accused the league of hiding the dangers of brain injuries linked to contact sports.

Tamura, 27, shot several people in the skyscraper’s lobby and another in a 33rd-floor office on Monday before he killed himself, authorities said. Among the victims were an off-duty New York City police officer and a security guard.

The attacker’s grievances with the NFL emerged as police began piecing together the details of his life and the cross-country road trip that brought him to Manhattan. It’s unclear if Tamura showed symptoms of CTE, which can only be diagnosed by examining the brain after a person dies.

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. 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

In the note, Tamura repeatedly said he was sorry and asked that his brain be studied for CTE. He mentioned a PBS Frontline documentary about the disease and 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.

NFL Commissioner Roger Goodell, who works out of the offices, called the shooting “an unspeakable act of violence in our building,” saying he was deeply grateful to the law enforcement officers who responded and to the one 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 hospitalized 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 Center. It’s also 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 corporate greed.

Surveillance video showed Tamura getting out of a BMW early Monday evening and strolling across a plaza in a button-down shirt and jacket with the rifle at his side before he entered the building, which also has offices for the investment firm Blackstone and other companies.

Once inside, he sprayed the lobby with gunfire, killing Didarul Islam, the off-duty police officer who was 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 someone there before fatally shooting himself, the commissioner said. He shot himself in the chest, according to Adams.