At least four people were killed and another critically injured after a lone gunman stormed an office tower in Midtown Manhattan and opened fire, before turning the gun on himself, officials say.

Police said that suspect Shane Tamura, 27, burst through the lobby of the skyscraper at 345 Park Avenue just before 6.30 p.m. on Monday, which includes corporate offices for the National Football League and the investment giant Blackstone.

Tamura, a former high school football star from Southern California, left Las Vegas on Saturday, Police Commissioner Jessica Tisch said at a news conference. He died in Rudin Management’s 33rd-floor offices from a self-inflicted gunshot wound to the chest, she said.

New York City police officer Didarul Islam, who was working in a private security role, was among those killed in the crossfire, Tisch added. A Blackstone executive was also shot dead in the tower’s lobby, sources told the Wall Street Journal. The NFL said one of its employees was critically injured.

Investigators allegedly found a “suicide note” in Tamura’s pocket claiming that he suffered from chronic traumatic encephalopathy (CTE), a disease linked to head trauma, and asked that his brain be studied, a source told CNN.