BOSTON (AP) -- Former New England Patriots star Aaron Hernandez had a severe case of the degenerative brain disease chronic traumatic encephalopathy, researchers said on Thursday. His lawyer announced a lawsuit against the NFL and the team, accusing them of hiding the true dangers of the sport.

Dr. Ann McKee, the director of the CTE Center at Boston University, said Hernandez had Stage 3 (out of 4) of the disease, which can cause violent mood swings, depression and other cognitive disorders .

"We're told it was the most severe case they had ever seen for someone of Aaron's age," attorney Jose Baez said.

Hernandez killed himself in April in the prison cell where he was serving a life-without-parole sentence for murder. Baez said Hernandez had shown signs of memory loss, impulsivity and aggression that could be attributed to CTE.

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

CTE, which can only be diagnosed in an autopsy, has been found in former members of the military , football players and boxers and others who have been subjected to repeated head trauma. A recent study found signs of the disease in 110 of 111 NFL players whose brains were inspected.

The lawsuit filed in U.S. District Court claimed that the league and Patriots failed to protect their players' safety, leading to the disease that deprived Hernandez's 4-year-old daughter, Avielle, of her father's companionship.

"Defendants were fully aware of the dangers of exposing NFL players, such as Aaron, to repeated traumatic head impacts," the lawsuit said. "Yet, defendants concealed and misrepresented the risks of repeated traumatic head impacts."

NFL spokesman Brian McCarthy said the league had not yet seen the lawsuit and could not comment. A Patriots spokesman did not immediately respond to a message seeking comment.

The league recently agreed to pay $1 billion to retired players who claimed it misled them about the dangers of playing football.

The "loss of consortium" lawsuit filed on Thursday is independent of the class-action suit that began making payments this summer. Baez said it was the first of its kind.

"If we have to be groundbreakers in this area, it's something we're prepared to do," he said.

Hernandez killed himself just hours before his former teammates visited the White House to celebrate their latest Super Bowl victory and a week after he 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.

Hernandez did not raise CTE in his defense at either trial because he claimed actual innocence.

"It's something I deeply regret," Baez said.

A star for the University of Florida when it won the 2008 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 semi-pro football player Odin Lloyd, who was dating the sister of Hernandez's fianc?e. Hernandez was convicted and sentenced to life in prison; the conviction was voided because he died before his appeals were exhausted, though that ruling is itself being appealed.