A gunman who killed four people when he stormed a skyscraper in the heart of New York on Monday evening left a note that appeared to blame the National Football League (NFL) for a brain injury, the city's Mayor Eric Adams has said.

The Manhattan high-rise shooter, a former high school football player, left a note referencing the brain disease CTE and appeared to have targeted the NFL’s offices, authorities said.

The man who killed four people in the New York City skyscraper that is home to NFL headquarters carried a note blaming the league for the cognitive problems he developed as a high school football player.