The progressive brain disease can be caused by repeated trauma to the head and has caused serious health problems for athletes who participate in contact sports, such as football.

Tamura killed four and badly wounded another before shooting himself in the chest.In the note, Tamura claimed to have CTE—chronic traumatic encephalopathy, a brain injury condition due to repetitive trauma to the brain—and that he wanted researchers to study his brain.

Chronic traumatic encephalopathy, or CTE, is a brain disease linked to repeated hits to the head, often seen in military veterans and athletes.