Both the NFL and the New England Patriots said they cannot comment on the lawsuit at this time.
Aaron Hernandez, the former NFL star who took his life while serving a murder sentence in prison, had chronic traumatic encephalopathy -- commonly known as CTE -- attorney Jose Baez said Thursday.
Baez cited study results compiled by the Boston University CTE Center, which examined Hernandez's brain for the neurodegenerative brain disease.
Hernandez played for the New England Patriots from 2010 to 2012 after a standout collegiate career at the University of Florida.
According to a statement from BU, Hernandez's brain was examined by Dr. Ann McKee, director of the CTE Center. The neurodegenerative brain disease has Alzheimer's-like symptoms, including memory loss, confusion, aggression, rage and, at times, suicidal behavior. It is believed to result from repeated trauma to the head, which results in a buildup of the abnormal protein tau that clumps in the brain.
The disease is categorized in four degrees, with Stage 4 being the most severe. According to McKee's analysis, Hernandez was found to have Stage 3 CTE, which is commonly associated with cognitive and memory loss, as well as behavioral changes and impaired judgment. Hernandez was found to have early degeneration of brain cells and large tears in the septum pellucidum, a central membrane of the brain.
The CTE Center statement added that he had "severe deposition of tau protein in the frontal lobes of the brain" as well as a "deposition of tau protein in nerve cells around small blood vessels, a unique feature of CTE."
The initial examination was confirmed by a second neuropathologist at Boston University.
CTE can be diagnosed only after death. Researchers are working on ways to detect it in the living, with the hope of being able to treat it one day.
Hernandez hanged himself in prison in April. He was 27.
"Aaron suffered from a severe case of CTE," Baez said. "Not only were the results positive, but we're told that it was the most severe case they had ever seen for someone of Aaron's age."
A representative for Boston University's CTE Center did not confirm to CNN that it was the most severe case it had seen and said, "We have nothing further to add."