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.

A gunman who killed four people inside a Manhattan office tower blamed his mental health problems on the National Football League and intended to target the league’s headquarters upstairs but took the wrong elevator,