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.

“CTE is linked to repetitive brain trauma and has a distinct pathology that can only be diagnosed at autopsy,” says Dr. Ross Zafonte, a principal investigator of the Football Players Health Study at Harvard University and executive vice dean at the University of Missouri.

The man who killed four people in the New York City skyscraper that is home to NFL headquarters carried a note blaming the league for his mental health problems.