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 man who killed at least four people in a N.Y.C. office building shooting claimed in his suicide note to have chronic traumatic encephalopathy (CTE). Here’s what to know about the degenerative brain disease.