Lawyers for Aaron Hernandez are suing the NFL and the New England Patriots over his death after a brain study showed the former football star suffered from a "severe case" of the degenerative brain disease chronic traumatic encephalopathy.

"Not only were the results positive, but we're told it was the most severe case they had ever seen for someone of Aaron's age," Hernandez's lawyer Jose Baez said in a news conference Thursday at his office.

The lawsuit was filed in federal court in Boston on behalf of Hernandez's 4-year-old daughter. It claims the team and the league deprived Avielle Hernandez of the companionship of her father.

Baez said Hernandez had Stage 3 out of 4 CTE, usually found in a 67-year-old man. Boston University officials who examined Hernandez's brain said he also had early brain atrophy and perforations in a central membrane. CTE can only be diagnosed in an autopsy.

Avielle Hernandez's mother, Shayanna Jenkins-Hernandez, attended Thursday's news conference but did not speak.

Jose Baez, the attorney representing the family of Aaron Hernandez, says the former football player suffered from the degenerative brain disease.

"We are deeply troubled, deeply saddened," Baez said. "And I know for a fact his entire family - especially Shayanna - is deeply troubled by this whole thing."

The Patriots did not immediately respond to requests for comment. The NFL said it has not seen a copy of the lawsuit and "cannot comment at this time."

An attorney representing Aaron Hernandez said Thursday that the former football star suffered from a serious brain injury.

CTE can be caused by repeated head trauma and leads to symptoms like violent mood swings, depression and other cognitive difficulties. Hernandez killed himself in April in the jail cell where he was serving a life-without-parole sentence for a 2013 murder. His death came just hours before the Patriots visited the White House to celebrate their latest Super Bowl victory.

Soon after his prison suicide in April, Hernandez's family decided they wanted his brain to be studied by the Boston University Chronic Traumatic Encephalopathy Center.

State officials originally refused to release the brain because it was part of the ongoing investigation into Hernandez's death, but later agreed to release it after the ex-NFL star's lawyer accused them of holding the brain illegally.

Ninety-nine percent of the 111 former NFL players whose brains were analyzed had CTE.

Baez said at the time that the family hoped Hernandez's brain could be examined to help future athletes and to shed any more light on his client's death. A recent study conducted by Boston University found CTE in the brains of 110 of the 111 brains of former NFL players that were tested.

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Hernandez, 27, was serving a life sentence for murder and was acquitted in two other killings just days before he hanged himself with a bed sheet attached to his cell window at the Souza-Baranowski Correctional Center in Shirley, Massachusetts, in the early morning hours of April 19.

Hernandez blocked access to his cell from the inside by jamming cardboard into the door tracks, investigators said. They also said there were no signs of a struggle and Hernandez was alone at the time of the hanging.

His Bible was found marked with blood at John 3:16, a verse that describes eternal life for those who believe in God. The verse name was also written in blood on the wall and in pen on his forehead.

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 fiancee. 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.

SUICIDE PREVENTION HELP: The National Suicide Prevention Hotline (1-800-273-8255) is open 24 hours a day, 7 days a week.