New York City is mourning NYPD Officer Didarul Islam and three other people killed in Monday's shooting inside an office building at 345 Park Ave.

The last killing of such a scale in the city was during a fast-food restaurant robbery in Queens in 2000. Mass murders of strangers are rare.

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.