New Delhi, July 29 -- The 27-year-old suspected gunman behind the deadly shooting at 345 Park Avenue in Midtown Manhattan reportedly left behind a multi-page suicide note pointing to chronic traumatic encephalopathy (CTE) as a key factor in his actions.

Shane D. Tamura, a former athlete, shot and killed four people - including a New York police officer - before turning the gun on himself on the 33rd floor of the 44-storey skyscraper on Monday. The building also houses the headquarters of the National Football League (NFL), which authorities believe Tamura deliberately targeted.

According to the New York Post, the note refers to his mental health struggles and links them to his past involvement in football. He reportedly blamed the sport...