NEW YORK (TNND) — Officials are still working to uncover what caused a former high school football player who claimed to suffer from a brain disease and blamed it on the NFL to carry out a deadly attack on a skyscraper in Midtown Manhattan.

The gunman shot and killed four people before turning the gun on himself on the 33rd floor of an office building that is the headquarters of the NFL, according to authorities.

After the attack, a handwritten note was found in his wallet, claiming he had chronic traumatic encephalopathy, known as CTE, and accused the league of covering up the dangers of the game, investigators said.

NYPD Commissioner Jessica Tisch said the man had a "documented mental health history."

Mayor Eric Adams told CBS Mornings on Tuesday that a preliminary investigation indicated the gunman took the wrong elevator bank up to the NFL headquarters and instead ended up at Rudin Management, "where he carried out additional shootings and took the lives of additional employees."

What happened?

Tisch said emergency crews were called to the Park Avenue office building Monday at around 6:30 p.m. for a report of someone shot.

The gunman was seen on surveillance video getting out of a black BMW with an M4 rifle before walking towards the skyscraper and entering the lobby, where he shot and killed 36-year-old police officer Didarul Islam, according to police.

He then fired his gun, shooting several others while walking towards the elevator and letting a woman walk out unharmed, Tisch said. The man continued up to the 33rd floor, where he fired more rounds, killing another person, before going down a hallway and shooting himself in the chest.

Law enforcement officials said they found him with a concealed carry permit for a handgun from Las Vegas.

At 7:52 p.m., Tisch said the scene had been contained and the lone shooter was neutralized.

Who were the victims?

Officer Islam was among the four people killed when he was working security in the building while off-duty.

He was an immigrant from Bangladesh who had worked in the police force for three and a half years, officials said. He is survived by two children and a pregnant wife.

“He made the ultimate sacrifice,” Tisch said. “Shot in cold blood, wearing a uniform that stood for the promise made to this city. He died as he lived — a hero.”

Financial firm Blackstone said it was "heartbroken to share that our colleague, Wesley LePatner, was among those who lost their lives in the tragic incident" in a memo to staff.

LePatner served as Global Head of Core+ Real Estate and the Chief Executive Officer of Blackstone Real Estate Income Trust (BREIT), according to Blackstone's website.

An unarmed security guard, Aland Etienne, was identified by the labor union SEIU 32BJ.

"This tragedy speaks to the sacrifice of security officers who risk their lives every day to keep New Yorkers and our buildings safe," the union President Manny Pastreich wrote on X. "Every time a security officer puts on their uniform, they put their lives on the line. Their contributions to our city are essential, though often unappreciated. Aland Etienne is a New York hero. We will remember him as such."

Julia Hyman, a 27-year-old Cornell graduate and employee of the Rudin Management Company, was called a "cherished Rudin colleague" by the Rudins.

Who was the gunman?

Shane Devon Tamura, 27, was identified as the shooter who had driven across the country from Nevada and arrived in New York City hours before the shooting, according to officials.

Law enforcement officials said they found Tamura with a concealed carry permit for a handgun from Las Vegas and that he had an overnight surveillance job at a casino called the Horseshoe Las Vegas.

After investigators searched Tamura's vehicle, they found a rifle case with rounds, a loaded revolver, ammunition and magazines, along with a backpack and medication, Tisch said.

Tisch said in a video statement on Wednesday that 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.

What is the motive?

Investigators said they are looking into whether Tamura specifically targeted NFL headquarters.

Authorities reported that in his note, he asked for his brain to be examined for CTE and wrote: “You can’t go against the NFL, they’ll squash you." It can only be diagnosed after a person's death.