Researchers at the Boston University School of Medicine said in a statement Thursday that Hernandez was dealing with chronic traumatic encephalopathy (CTE).
Dr. Ann McKee, the director of the school's CTE Center and Chief of Neuropathology at the VA-BU-CLF Brain Bank, said Hernandez had Stage 3 CTE, with Stage 4 the most severe of the disease that has plagued numerous NFL players.
Hernandez's lawyer, Jose Baez, told Deadspin.com that his law firm filed a federal lawsuit against the NFL and the Patriots on behalf of Hernandez's daughter and Shayanna Jenkins-Hernandez, who was engaged to the player at the time of his death.
The lawsuit said Hernandez had "the most severe case of Chronic Traumatic Encephalopathy (CTE) medically seen in a person of his young age of 28 years" by Boston University researchers.
According to the filing, Hernandez's Stage 3 CTE is "usually seen in players with a median age of death of 67 years."
The suit charges that the defendants were "fully aware" of the damage that could be inflicted to Hernandez's brain by the time he entered the NFL in 2010 and "failed to disclose, treat, or protect him from the dangers of such damage."
Hernandez, who played three seasons with the Patriots, died after hanging himself in his cell on April 19. He was serving a life sentence for the murder of Odin Lloyd, a sentence delivered in April 2015.
The suicide came four days after Hernandez was acquitted of a double murder four days in the resolution of a separate case.
Due to a Massachusetts state law, a judge in early May overturned Hernandez's murder conviction on a technicality. The Massachusetts Supreme Judicial Court procedurally voids first-degree murder convictions in cases when a defendant dies before their appeal is heard.
Hernandez was found guilty of first-degree murder after the 2013 killing of Lloyd, a semi-professional football player who was dating Hernandez's fiancee's sister. Lloyd's body was found in an industrial park about one mile away from Hernandez's home in North Attleboro, Mass.
Hernandez also was charged with murder in the 2012 fatal shootings of Daniel de Abreu and Safiro Furtado in Boston. He was acquitted of those charges in mid-April but convicted of unlawful possession of a gun and sentenced to four to five years in prison.
A fourth-round draft pick in 2010, Hernandez signed a seven-year, $40 million contract with the Patriots prior to the 2012 season. That deal included a $12.5 million signing bonus and $15.9 million guaranteed.