A New York City police officer and three other people were killed in a shooting in Midtown Manhattan on Monday evening, police confirmed at a late-night news conference.

Investigators are examining why the gunman who killed four people before taking his own life at a New York City office tower targeted the National Football League and whether he suffered from a brain disease.

Didarul Islam, a 36-year-old NYPD officer, was off duty and working a security job when he was fatally shot by a gunman who opened fire at the 345 Park Avenue office building on Monday.