A gunman donned body armor and wielding an M4 assault rifle stormed into a Midtown Manhattan office building on Monday, where he opened fire, resulting in four fatalities before turning the gun on himself.

Among the victims was a New York police officer who was off duty and working as a security guard at the building, WABC reported. Another individual was injured and is in critical but stable condition.

The incident occurred around 6:30 p.m. in the Park Avenue office building which is home to the headquarters of the investment firm Blackstone and the National Football League.

The suspect was identified as Shane Tamura, 27. Sources told ABC News that he carried a rambling, 3-page note in his pocket saying he suffered from Chronic Traumatic Encephalopathy (CTE), a progressive brain disease caused by repeated head trauma. The note asked that his brain be studied and made references to the NFL, the sources said.

Police said Tamura played high school football. He is not listed in a database of NFL players.

New York Police Commissioner Jessica Tisch said Tamura barricaded himself on the 33rd floor of the towering office building, where he was found dead.

Tamura, from Las Vegas, had a license to carry a concealed weapon in Nevada, according to Tisch, who said he traveled across the country before carrying out the shooting. She said he travelled through Colorado on Saturday, Nebraska and Iowa on Sunday, and was in Columbia, New Jersey, just before 4:30 p.m. on Monday, according to WNBC.

He double parked his BMW on the street and got out, carrying the M4. Surveillance footage captured him striding across the plaza to the building, where he entered the lobby and immediately opened fire, spraying he area with bullets and hitting off-duty police officer Didarul Islam, 36, who was in uniform at his security job, WABC reported.

Then, Tisch said, he went to the elevator and rode up to the 33rd floor, where he shot another person and then himself.

Investigators don’t yet know why he went to that floor, which houses the offices of a real-estate company.

Islam, the police officer who was killed, was a Bangladeshi immigrant who had been on the New York police force for four years. He leaves behind a wife and two young children. His wife is pregnant with their third child.

“Early tonight, I met with the officer’s family. I told them that he was a hero, and we admire him for putting his life on the line,” said Mayor Eric Adams.

The investment firm Blackstone confirmed that one of its executives, Wesley LePatner, was killed in the shooting, WNBC reported.

“She was brilliant, passionate, warm, generous, and deeply respected within our firm and beyond,” the company said. “She embodied the best of Blackstone. Our prayers are with her husband, children and family. We are also saddened by the loss of the other innocent victims as well, including brave security personnel and NYPD.”

NFL Commissioner Roger Goodell said in a note to the league’s employees that one of its employees was the victim who survived the shooting. He called the shooting an “unspeakable act of violence” and said there would be increased security at the building, even though officials said Tamura appeared to be acting alone and there was no further threat to the public.

Four other people suffered minor injuries trying to flee the shooting.