A degenerative brain disease that has been linked to concussions and other head trauma common in military combat and contact sports, CTE has been diagnosed in more than 100 former NFL players and arisen as an existential threat to the United States’ most powerful pro sports league.

After killing four and taking his own life, Shane Tamura — a former football player at two L.A.-area high schools — left behind a suicide note, authorities say, alleging he had the brain disease CTE.