Shane Tamura, the 27-year-old gunman who fatally shot 36-year-old New York City police officer Didarul Islam and several others in a New York City skyscraper on Monday, was reportedly found dead with a note claiming that football had given him chronic traumatic encephalopathy, or CTE.

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

CTE is defined as a progressive neurodegenerative disease that causes brain damage similar to what’s seen in patients with Alzheimer’s disease, per the National Institutes of Health (NIH). There is a known link between repeated head trauma, contact sports, and the development of CTE.