Shane Tamura — who fatally shot four people at a Midtown East office building — said in a suicide note he believed he suffered from CTE and requested his brain be studied.

The degenerative brain disease that has besieged the National Football League for two decades with a billion-dollar lawsuit, congressional hearings, an A-list movie and an unrelenting cortege of

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.