Researchers have diagnosed many former football players with CTE since 2005. It can only be diagnosed through a brain autopsy after the patient dies.

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

The gunman who opened fire in midtown Manhattan on Monday, killing four people before turning the gun on himself, left a note mentioning “CTE” several times, officials familiar with the investigation told NBC News.