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 his mental health problems.

The gunman in the deadly Midtown Manhattan shooting, Shane Tamura, wrote about the brain disease CTE in a note obtained by investigators.

Chronic traumatic encephalopathy, or CTE, is a brain disease linked to repeated hits to the head, often seen in military veterans and athletes including football players, hockey players and boxers. It causes symptoms, which usually appear years after the head trauma, including memory loss, mood changes, confusion and trouble thinking clearly.