Grimsley died in February at 45 after he shot himself in the chest in what police ruled an accident. Subsequent analysis of his brain tissue confirmed the presence of neurofibrillary tangles that had already begun to affect Grimsley’s behavior and memory, said Dr. Ann C. McKee, an associate professor of neurology and pathology at the Boston University School of Medicine and a co-director of the new brain-study center.

Image John Grimsley, right, was found to have chronic traumatic encephalopathy. Credit... Lou Witt/NFL

Of the six former N.F.L. players’ brains that have been examined in this manner, Grimsley’s was the fifth to be found to have chronic traumatic encephalopathy, joining the former Philadelphia Eagles defensive back Andre Waters and the former Pittsburgh Steelers Mike Webster, Terry Long and Justin Strzelczyk. (The condition can be confirmed only by post-mortem tissue analysis; X-rays and magnetic resonance imaging tests cannot yet detect it.) Because each player died relatively young, from 36 to 50, they provided an opportunity to examine brain abnormalities that are exceedingly rare in someone of that age without a history of repetitive brain injury.

The only former N.F.L. player whose brain did not show C.T.E. was the former running back Damien Nash, who died last year at 24 after collapsing while playing basketball. C.T.E., believed to result from trauma endured over many years, has almost never been found in anyone that young.

“I’ve seen thousands of brains of individuals with neurogenerative diseases and debilitating diseases,” McKee said. “I can say this is identical to the pugilistica dementia that I’ve seen in boxers in their 70s and 80s. It’s milder because the patients are younger. But once triggered, it seems to progress. The people that develop this disease, most of them show symptoms 10 or 20 years after retirement. It progresses inexorably until death.”

As each case of C.T.E. in a former player has come to light, N.F.L. officials have contended that they are isolated incidents from which no conclusions can be drawn, often because there are no documented medical histories of the players’ concussions in the N.F.L. or otherwise. Players at all levels of football are known to not reveal their concussions for fear of being removed from games or being seen as weak.

At a league conference on concussions in June 2007, Commissioner Roger Goodell pointed to the lack of concussion documentation from Strzelczyk’s career and said, “There’s no record he may have had a concussion swimming.” He added: “A concussion happens in a variety of different activities.”