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

Shane Tamura, who police said shot and killed four people in Manhattan, claims he had chronic traumatic encephalopathy.

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.