BOSTON – 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 ex-players’ obituaries intruded on America's favorite sport in the most violent manner yet when a gunman who turned out to have the disease killed four people earlier this year.

At the time of the July 28 attack, Shane Tamura, the Las Vegas casino worker who targeted the New York City skyscraper that is home to the NFL’s headquarters, carried a note blaming the league for his mental health problems.

Tamura, 27, who played football in high school, said in a three-page note found in his wallet that he had chronic traumatic encephalopathy — diagnosable only after death — and implored t

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