The National Football League on Friday vowed to vigorously fight a lawsuit filed on behalf of former New England Patriots star Aaron Hernandez's family that claimed his severe chronic traumatic encephalopathy led to his suicide.

Hernandez, 27, hung himself in April in a Massachusetts jail where he was serving a life sentence for the 2013 murder of an acquaintance. His death stunned his friends and fans, coming just five days after he was acquitted of a separate 2012 double homicide.

The lawyer who secured his that acquittal, Jose Baez, on Thursday sued the league on behalf of Hernandez's 4-year-old daughter and fiancee, Shayanna Jenkins, saying a severe case of the brain disease CTE that Hernandez developed as a player contributed to his death. The suit seeks $20 million.

Researchers on CTE at Boston University diagnosed the brain condition after Hernandez's death based on an autopsy.

CTE, which currently can be diagnosed only in a dead person's brain tissue, is linked to repeated head trauma and can lead to aggression and dementia. It has been found in athletes including Hall of Fame linebacker Junior Seau and Pro Bowl safety Dave Duerson, both of whom committed suicide.

"We intend to contest the claim vigorously," NFL spokesman Joe Lockhart told reporters on a conference call. "Any attempt here to paint Aaron Hernandez as a victim, we believe is misguided. His personal story is complex and doesn't lend itself to simple answers."

Hernandez's two murder trials painted a picture of a young man who witnesses said regularly used illegal drugs and frequently worried about being challenged and disrespected by strangers. He was found guilty in 2015 of murdering acquaintance Odin Lloyd in an industrial park near his home.

Earlier this year he was found not guilty of fatally shooting two men outside a Boston nightclub in 2012. The prior conviction on Lloyd's killing was vacated by a judge following Hernandez's death due to a quirk in Massachusetts law that allows such a move if a defendant has not exhausted all possible avenues of appeal.

Prosecutors have challenged that judge's decision.

Lockhart said the league changed more than 50 rules intended to reduce the risk of CTE in football. Those include more sideline tests of players and banning the most dangerous head-to-head hits.

Hernandez had a $41 million contract with the Patriots when he was arrested in June 2013 and charged with Lloyd's murder.

(Reporting by Scott Malone; Editing by Cynthia Osterman)