For the Hoffmans, it was the answer to the mystery of Ryan’s continuing problems, which included jail time and joblessness — troubles that defined his life after a successful college career on a team ranked in the top 10.

“I wanted to know exactly what happened to my brother, and I just knew football did it,” his sister, Kira Soto, said Monday during a phone call as she began to cry. “I’ve been looking into this for 15 years and defended him when people said it was just the drugs and judged him for something he couldn’t help, something that he struggled with. Well, we know now. We know.”

Dr. Ann McKee, the chief of neuropathology at the V.A. Boston Healthcare System and professor of neurology and pathology at Boston University School of Medicine, examined Hoffman’s brain and told me he had Stage 2 C.T.E., on her scale of 0 to 4. It was the same severity found in the brain of Tyler Sash, the former Giants safety who in September died of an accidental drug overdose, at 27, and in the brain of Junior Seau, the retired 12-time Pro Bowl linebacker who shot and killed himself in 2012 at 43.

Dr. McKee said that Stage 2 C.T.E. can be especially symptomatic, causing problems like depression, short-term memory loss, lack of impulse control, irritability and mood swings, and that some symptoms could be confused with mental illness. Last year, Hoffman told me that he had received diagnoses of various mental illnesses, including manic depression, but that medications never seemed to help.