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.

There were 23 shell casings and 13 bullet fragments recovered from the lobby of 345 Park Avenue and 24 spent shell casings on the 33rd floor, police said.