New York City Mayor Eric Adams said on Monday there was an "active shooter" investigation underway in central Manhattan, while the FBI added it was also providing support at what it called an active crime scene.

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.