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.

The gunman in a recent Manhattan shooting claimed he had CTE. Here's what CTE is, plus what the most recent research suggests.