Doctors at Boston University who studied the brain of former Patriots tight end Aaron Hernandez determined that Hernandez had Stage 3 chronic traumatic encephalopathy, which is one of the most severe forms of CTE, according to the New York Times.

Hernandez was convicted of murdering his former friend, Odin Lloyd. He committed suicide in prison while serving a life sentence without the possibility of parole. His conviction was voided following his death because Hernandez had not yet exhausted his options for appeal, which is consistent with Massachusetts law. That voided conviction is also under appeal.

Jose Baez, part of Hernandez’s legal team, said during a press conference on Thursday that he will file a federal lawsuit on behalf of Hernandez’s daughter against the Patriots and the NFL over the diagnosis. Baez said that researchers identified Hernandez’s condition as “the most severe case they had ever seen in someone of Aaron’s age.” Hernandez was 27 at the time of his death, and hadn’t played football since the 2012 season.

During a press conference on Friday, NFL spokesman Joe Lockhart cautioned against viewing Hernandez as a victim.

“His personal story is complex; it doesn’t lend itself to simple answers,” Lockhart said, via Pro Football Talk’s Mike Florio.

Lockhart also said that the NFL plans to contest the lawsuit filed by Hernandez’s legal team “vigorously.”

CTE does not just stem from concussions, but can result from any kind of repeated trauma to the head. It is most commonly diagnosed in military veterans and athletes who play contact sports.

The disease can only be diagnosed posthumously at this time through the study of brain tissue. Symptoms of CTE vary from person to person and often mirror other conditions, which makes diagnosis in a living person a challenge. A person with CTE can exhibit a combination of symptoms, including confusion, depression, memory loss, poor impulse control, anxiety, trouble controlling anger, and suicidal thoughts and tendencies.

Boston University published a study in July 2017 that found that 99 percent of the brains of former NFL players submitted for research tested positive for CTE. The authors of the study acknowledged there may be some degree of bias involved. Former players and their loved ones chose to donate their brains because they exhibited symptoms of the disease, and it was a small sample size.

Hernandez was a standout at Bristol Central High School in Connecticut, and then played at the University of Florida for three seasons. He spent three seasons with the Patriots before being released by the team after his arrest for Lloyd’s murder.