The brain of late New England Patriots tight end Aaron Hernandez showed advanced signs of the degenerative brain disease chronic traumatic encephalopathy, his lawyer said Thursday.

In a news conference at his offices, Jose Baez announced that he had filed a federal lawsuit against the Patriots and the National Football League on behalf of Hernandez's young daughter.

Hernandez, who was convicted of first-degree murder in the 2013 shooting death of Boston semipro football player Odin Lloyd, was found hanged in his Massachusetts jail cell this past April.

At the time of his death, the lawsuit states, "Aaron had Stage III CTE usually seen in players with a median death age of 67 years." Hernandez was 27 years old when he died.

Dr. Ann McKee, the director of the CTE Center at Boston University, concluded that Hernandez also had early brain atrophy and large perforations in a central membrane.

CTE, which can only be diagnosed posthumously, can be caused by repeated head trauma and leads to symptoms like violent mood swings, depression and other cognitive difficulties. A recent study found evidence of the disease in 110 of 111 former NFL players whose brains were examined.

CTE has been linked with repeated concussions and involves brain damage particularly in the frontal region that controls many functions including judgment, emotion, impulse control, social behavior and memory.

Baez claims that the Patriots and the league were negligent and failed to "disclose, treat or protect" Hernandez from so-called "repetitive impact injuries." The lawsuit seeks unspecified damages and is separate from a $1 billion settlement in which the league agreed to pay families of players who suffered brain damage because of repeated head trauma while playing football.

When contacted by Fox News, Patriots spokesman Stacey James said he did not anticipate the team would comment on the lawsuit. When reached by email, NFL spokesman Brian McCarthy wrote, "We have not seen the lawsuit and cannot comment at this time."

A star for the University of Florida when it won the 2008 national title, Hernandez dropped to the fourth round of the NFL draft because of trouble in college that included a failed drug test and a bar fight. His name had also come up in an investigation into a shooting.

In three seasons with the Patriots, Hernandez joined Rob Gronkowski to form one of the most potent tight end duos in NFL history. In 2011, his second season, Hernandez caught 79 passes for 910 yards and seven touchdowns to help the team reach the Super Bowl, and he was rewarded with a $40 million contract.

But the Patriots released him in 2013, shortly after he was arrested in the killing of Lloyd, who was dating the sister of Hernandez's fiancée. Hernandez was convicted and sentenced to life in prison; the conviction was voided because he died before his appeals were exhausted, though that decision is itself being appealed.

A week before his suicide, Hernandez was acquitted in the 2012 drive-by shootings of two men in Boston. Prosecutors had argued that Hernandez gunned the two men down after one accidentally spilled a drink on him in a nightclub, and then got a tattoo of a handgun and the words "God Forgives" to commemorate the crime.

Baez said he deeply regretted not raising the issue of Hernandez having CTE during his murder trials. He said they did not blame CTE for the murders because Hernandez's defense was actual innocence.

"When hindsight is 20-20, you look back and there are things you might have noticed but you didn't know," he said.

The Associated Press contributed to this report.