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 says, BBC reported.

The attacker, 27-year-old Shane Tamura of Las Vegas, shot himself dead after opening fire in a building where the American football league has its headquarters, but went to a different part of the building after taking the wrong lift.

The gunman was carrying a note in which he blamed CTE, a brain disease triggered by head trauma, for his mental illness, Adams said.

Tamura played football as a teenager but did not play in the NFL, ex-teammates have told US media, according to BBC.