The killing of four people including a police officer in a Midtown Manhattan office building rattled a city where mass attacks are rare and violent crime has declined.

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.

No motive has been announced. A suicide note was found expressing grievances with the NFL and saying Tamura suffered from CTE, a source said. Follow for live updates.