Aaron Hernandez, the former New England Patriots tight end who committed suicide in April while serving a life sentence for murder, was found to have a severe form of C.T.E., the degenerative brain disease linked to repeated head trauma that has been found in more than 100 former N.F.L. players.

researchers at Boston University who examined the brain determined it was “the most severe case they had ever seen in someone of Aaron’s age,” which was 27.

Researchers who examined the brain determined it was “the most severe case they had ever seen in someone of Aaron’s age,” said a lawyer for Hernandez in announcing the result at a news conference on Thursday. Hernandez was 27.

C.T.E., or chronic traumatic encephalopathy, can be diagnosed only posthumously. Hernandez is the latest former N.F.L. player to have committed suicide and then been found to have C.T.E., joining Dave Duerson, Junior Seau, Andre Waters, Ray Easterling and Jovan Belcher, among others. Seau and Duerson shot themselves in the chest apparently so that researchers would be able to examine their brains. Hernandez was found hanging in his prison cell.

Seau, Duerson and Waters were all older than 40, while Hernandez is one of the youngest former N.F.L. players to have been found with the disease. In July, researchers at Boston University released findings that showed that they had found C.T.E. in the brains of 110 of the 111 former N.F.L. players they had examined.

Jose Baez, Hernandez’s lawyer, said Hernandez’s brain showed a level of damage that was seen in players with a median age of 67 years.

Baez said he has filed a lawsuit in federal court against the Patriots and the N.F.L. on behalf of Hernandez’s daughter, Avielle. The suit seeks unspecified damages for loss of parental support. The suit alleges that the Patriots and the league were “fully aware of the damage that could be inflicted from repetitive impact injuries and failed to disclose, treat, or protect him from the dangers of such damage.”

Baez said he did not rule out adding the N.C.A.A. or the University of Florida, where Hernandez played college football, to the complaint.

Baez said that in hindsight, his family had witnessed Hernandez act in ways that were consistent with a person found to have C.T.E., “but you don’t know.”

Hernandez’s brain was examined by Dr. Ann McKee, chief of neuropathology at the VA Boston Healthcare System and director of the CTE Center at Boston University. She developed categories to describe the severity of the disease. Those with Stage 3 of C.T.E., typically had cognitive impairment and trouble with executive functions like planning and organizing. Those with Stage 4, the most severe version of the disease, had dementia, difficulty finding words and aggression.

Dr. McKee said in a statement that Hernandez had Stage 3, and that he had “early brain atrophy.”

The discovery of C.T.E. adds another turn in Hernandez’s meteoric rise and fall. After a standout career at Florida, Hernandez signed a record $40 million contract with the Patriots in 2012, when he was 22. Just five years before, he had been working menial jobs in his hardscrabble hometown of Bristol, Conn., where he drove a $300 used car he bought with money borrowed from friends.

At the University of Florida, he helped the Gators win the national title in 2008. But he fell to the fourth round of the N.F.L. draft because of off-field issues including involvement in a bar fight.

Yet 10 months after he signed his contract with the Patriots, in 2013, the body of a friend who had been shot multiple times was discovered. He was convicted of the friend’s murder, was accused and acquitted of two other killings from 2012 and became a stark example of N.F.L. players who exhibit violence off the field.

Even his demise was filled with turmoil. After Hernandez died, Baez called a news conference in front of the Office of the Chief Medical Examiner and accused the state of illegally withholding Hernandez’s brain. Hernandez’s body had been discovered the day before tied with a bedsheet to the window of his prison cell in Shirley, Mass. His death was later ruled a suicide.