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.

The 27-year-old man who allegedly shot and killed four people at a Midtown Manhattan office building on Monday carried a note in his pocket claiming he suffered from the disease, police sources told ABC News, though there is no evidence so far to confirm he had the condition.

Suspect Shane Tamura died by suicide after the Monday mass shooting at 345 Park Avenue, police said. Tamura, 27, played high school football in Los Angeles but did not play p

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