CTE cannot be diagnosed until the patient has died and their brain examined. But doctors said there are symptoms such as changes in cognition, mood and behavior. The American Brain Foundation said they can include difficulty thinking, short- and/or long-term memory loss, depression, anxiety, paranoia, headaches and suicidal thoughts or behavior.

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 27-year-old man who shot and killed four people at a Midtown office building on Monday carried a note in his pocket, police sources told ABC News.