Aaron Hernandez had a severe case of the degenerative brain disease chronic traumatic encephalopathy, researchers said on Thursday.
An attorney for Hernandez announced a lawsuit against the NFL and the New England Patriots for hiding the true dangers of the sport.
Hernandez had stage 3 (out of 4) of the disease, which can cause violent mood swings, depression and other cognitive disorders.
"We're told it was the most severe case they had ever seen for someone of Aaron's age," attorney Jose Baez said.
Hernandez was 27 when he killed himself in April.
Baez said Hernandez had shown signs of memory loss, impulsivity and aggression that could be attributed to CTE.
"When hindsight is 20-20, you look back and there are things you might have noticed," he said. "But you don't know."