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.

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.

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.