Former New England Patriots tight end Aaron Hernandez had one of the most severe cases of CTE researchers have encountered in someone his age, a lawyer representing his daughter told a news conference on Thursday.
Boston University's (BU) centre for the study of chronic traumatic encephalopathy announced on Thursday that an examination of Hernandez's brain indicated he had Stage 3 CTE, one step short of the most severe level.
The 27-year-old killed himself in prison in April while serving a life sentence for murder.
Attorney Jose Baez said he is filing a federal lawsuit against the NFL and the Patriots on behalf of Hernandez's daughter.
"The results of Aaron Hernandez's tests were positive. Aaron suffered from a severe case of CTE," said Baez.
"Not only were the results positive but we are told it was the most severe case they have seen for someone of Aaron's age."
A statement on the BU centre's website read: "A neuropathological examination of Aaron Hernandez's brain was conducted by Dr Ann McKee, professor of pathology and neurology at Boston University School of Medicine, Director of BU's CTE Center and Chief of Neuropathology at the VA-BU-CLF Brain Bank.
"Based on characteristic neuropathological findings, Dr McKee concluded that Mr Hernandez had chronic traumatic encephalopathy (CTE), Stage 3 out of 4, (Stage 4 being the most severe). This diagnosis was confirmed by a second VABHS neuropathologist. In addition, Mr Hernandez had early brain atrophy and large perforations in the septum pellucidum, a central membrane.
"Dr McKee and the BU CTE team have extensive experience in the diagnosis of CTE and have contributed landmark publications on traumatic brain injury and CTE in athletes and veterans. Her research has demonstrated that CTE is associated with aggressiveness, explosiveness, impulsivity, depression, memory loss and other cognitive changes.
"We are grateful to the family of Aaron Hernandez for donating his brain to the VA-BU-CLF brain bank."