For now, C.T.E. can only be diagnosed in autopsies. Several former star players, including Junior Seau, Dave Duerson and Andre Waters, committed suicide and were later found to have C.T.E. Although those players were all older than 40, the disease has been found in former athletes in their 20s.

The body of Hernandez, 27, was discovered early Wednesday, tied with a bedsheet to the window of his prison cell in Shirley, Mass. Joseph D. Early Jr., the district attorney for Worcester County, released a statement on Thursday that said the cause of death was asphyxia by hanging.

Hernandez, who had been acquitted days earlier of a 2012 double murder, had been alone in his cell since 8 p.m. Tuesday, the statement said. His body was discovered about seven hours later, along with a Bible and three handwritten notes.

Earlier on Thursday, Baez, who represented Hernandez during the double murder trial, said it was too soon to determine whether his death was a suicide.

“We don’t make that call,” Baez said, “until everything is done.”

Baez said Hernandez’s family wanted to donate his brain to the C.T.E. Center at Boston University, which has studied the disease for years. The center, which has an extensive brain bank, has produced much of the leading research on the disease and its link to repeated head trauma.