BOSTON — The suicide of the former Chicago Bears star Dave Duerson became more alarming Monday, when Boston University researchers announced that his brain had developed the same trauma-induced disease recently found in more than 20 deceased players.

The diagnosis adds a new and perhaps pivotal chapter to football’s still-unfolding narrative surrounding concussions. Duerson shot himself Feb. 17 in the chest rather than the head, presumably so that his brain could be examined by Boston University’s Center for the Study of Traumatic Encephalopathy, which announced its diagnosis.

About two dozen retired N.F.L. players have been found to have the disease, known as chronic traumatic encephalopathy, but none acted upon his suspicion of it like Duerson, 50, who complained to family of his deteriorating mental state during his final months.

His death reminded the football community that for all the reform in the management of concussions and other on-field brain trauma in recent years, the damage to past players remains a vestige of the game’s more brutal times.