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.

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.

Shane Tamura is suspected of shooting four people in Midtown Manhattan, where the NFL offices are. A note he left references the NFL and CTE.