Aaron Hernandez, the former New England Patriots player who committed suicide in April while serving a life sentence for murder, suffered from chronic traumatic encephalopathy, the neurodegenerative disease linked to bruising hits on the football field, according to a Boston University study of his brain.

The findings prompted a federal lawsuit against both the NFL and Hernandez’s former team.

The 27-year-old had the second-most severe form of the disease and one of the most advanced cases that researchers had seen in a subject his age.

“Aaron had Stage III CTE usually seen in players with median age of death of 67 years,” according to a lawsuit filed Thursday in U.S. District Court.

Hernandez’s attorney, Jose Baez, filed the complaint in U.S. District Court on behalf of the former player’s 4-year-old daughter. The lawsuit, which seeks $20 million in damages, states that because of the NFL and the Patriots’ conduct, Avielle Hernandez was “deprived of the love, affection, society and companionship of her father while he was alive.”

Joe Lockhart, the NFL’s executive vice president of communications and public affairs, said Friday the league believes that the lawsuit “will face significant legal issues from the start” and the NFL intends to contest it “vigorously.”

The lawsuit does not link Hernandez’s crimes with the disease, which is associated with aggressiveness, erratic behavior, depression, suicidal thoughts and other cognitive issues, but does state he “succumbed to the symptoms of CTE and committed suicide.”

Hernandez, who had been serving time for the 2013 killing of Odin Lloyd, hanged himself in his Massachusetts cell April 19, five days after he was acquitted in of the 2012 murders of Daniel de Abreu and Safiro Furtado.

The lawsuit says the Patriots performed preseason exams of Hernandez and should have recognized signs of cognitive impairment. The NFL was “fully aware of the damage that could be inflicted from repetitive impact injuries and failed to disclose, treat or protect him from the danger of such damage,” the complaint states.

Lockhart, in a Friday conference call with reporters, called “any attempt to paint Aaron Hernandez as a victim” misguided.

“There are a number of issues,” Lockhart said. “We just saw this last night. At first blush, there are several things that jump out.”

The league will review whether the lawsuit is precluded by the NFL’s settlement with former players who sued over the health effects of concussions, Lockhart said. He declined to discuss other issues raised by the lawsuit in detail.

“I think ‘fight vigorously’ is clear,” Lockhart said.

The Patriots declined to comment Thursday evening.

Hernandez’s professional career was not a long one. He played 44 NFL games, including the playoffs, in three seasons for the Patriots before he was arrested at age 23. He also played in 40 games in three seasons at the University of Florida, but research has found that football players suffer from CTE regardless of how long they play the game or what level they reach. Recent studies suggest that the accumulation of smaller hits can damage the brain as much — if not more — as the head-rattling collisions that might result in concussions.

The study results released Thursday from Boston Unversity’s CTE Center also showed Hernandez suffered from early brain atrophy and had large perforations in his septum pellucidum, a major membrane in the brain associated with cognitive development.

According to a statement from the CTE Center, a second neuropathologist from VA Boston Healthcare System confirmed the diagnosis.

“Everyone, including and especially his family, is deeply troubled by this whole thing,” Baez told reporters at an afternoon news conference.

Researchers at Boston University have been at the forefront of CTE research in recent years. In a study published in July, researchers studied the brains of men who played football at all levels and found evidence of CTE in 177 of 202 — 87 percent — of them. The disease was evident across all levels of play, but the highest percentage was found among those who reached the NFL — 110 of 111 brains examined. Researchers cautioned that there were limitations to the study and the brains studied were mostly donated by concerned family members, meaning they might not necessarily be representative of all former football players.

Hernandez is not the first high-profile CTE case, but it could be one that resonates with both fans of the NFL and parents of youth players. Recent studies have shown that playing football before age 12 could result in long-term mood and behavioral issues. Some of the most high-profile CTE cases involved players who committed suicide, such as Junior Seau, Dave Duerson and Andre Waters, but Hernandez marks the first former NFL player who was diagnosed after being convicted of a heinous crime.

Even though CTE can only be diagnosed after death, Baez, the attorney, said Thursday he wished Hernandez had undergone more extensive testing while still alive.

“I will say for lawyers representing athletes of contact sports, that this is something that you should definitely consider having your clients take some of the presumptive tests that are available,” he said. “It is something that we considered as a team, but did not pursue simply because his defense was one of actual innocence and not one of mitigation. It’s something I deeply regret.”

Officials at the Souza-Baranowski Correctional Center in Shirley, Mass., discovered Hernandez’s body with a bedsheet tied around his neck shortly after 3 a.m. on April 19, but it took more than a day for state officials to rule the death a suicide. This caused a brief controversy surrounding Hernandez’s remains, including his brain, which the family had requested be sent to Boston University’s CTE Center for further study.

The brain eventually found its way to the brain bank and onto the table of Ann McKee, a leading researcher in the field who has authored numerous CTE studies. Hernandez marks just her latest high-profile diagnosis.

“We are grateful to the family of Aaron Hernandez for donating his brain to VA-BU-CLF brain bank,” the school’s CTE Center said in its statement.

CTE, a brain degeneration disease, has been found in the brains of deceased NFL Hall of Famers. Here's what you need to know about it. (Monica Akhtar/The Washington Post)

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