The daughter of former New England Patriots player Aaron Hernandez, who killed himself in April after being acquitted in his second murder trial, has sued the league and the team after tests revealed her father had a "severe case" of the brain disease chronic traumatic encephalopathy.

Relatives of the 27-year-old former athlete had asked that his brain be tested for CTE after his body was found hanging in a Massachusetts prison where he was serving a life sentence for the 2013 murder of an acquaintance.

Researchers at Boston University, the leading centre studying CTE, assessed Hernandez's brain, said attorney Jose Baez, who successfully defended the athlete in a double-murder case this year.

"It was the most severe case they had ever seen," Baez told reporters in Boston. "It was an advanced stage."

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