Aaron Hernandez's family suing NFL, Patriots

Doctors at Boston University determined Hernandez, a former New England Patriots star who committed suicide in prison, had an advanced form of the brain disease CTE.
3:33 | 09/22/17

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Transcript for Aaron Hernandez's family suing NFL, Patriots
with that major headline about Aaron Hernandez. A lawyer for the late NFL player and convicted murderer now says researchers found what they called the most severe case of cte they had ever seen in someone his age. ABC's linsey Davis is here with the story this morning. Good morning, linsey. On the heels of that the family filed a $20 million lawsuit to compensate his 4-year-old daughter for the loss of her father. The allegation is that both the NFL and the patriots not only knew but hid the dangers of the sport. That was complete. This is Hernandez. Reporter: Doctors now say former NFL superstar Aaron Hernandez suffered from chronic traumatic encephalopathy, commonly known as cte. Not only were the results positive but we're told that it was the most severe case they had ever seen in someone -- for someone of Aaron's age. Reporter: Doctors at the Boston university cte center examined Hernandez's brain concluding that the former new England patriots tight end was suffering from an advanced form of the brain disease, stage 3 out of 4. These results come five months after the 27-year-old committed suicide in prison while serving a life sentence for the 2013 murder of his friend Odin Lloyd. Guilty of murder in the first degree. Reporter: Now his family is taking aim at the NFL and his former team, the new England patriots, saying in a lawsuit, defendants were fully aware of the dangers of exposing NFL players such as Mr. Hernandez to repeated traumatic head impacts and blaming them for his death. Claiming depression, uncontrollable aggression and suicidal impulses are recognized to symptoms of late stage cte. They finalized a $1 billion class action by former players who claim they suffered traumatic brain injuries during their career. No comment just yet from the NFL or the patriots. Hernandez was just 27 years old. Yet, researchers say the damage he sustained was similar to that of players in their 60s. The lawyer says the family is also considering a lawsuit against the NCAA and his college team, the university of Florida. David. All right, linsey, our thanks to you. I want to bring in Dr. Jennifer Ashton and, Jen, we have been talking about this and known videotape is an issue for so long. When you heard doctors say this is the most severe case in someone this age ever, what does it tell you. Cte, neurodegenerative brain disorder caused by repeated head trauma causing symptoms of memory loss, aggression, depression, in some cases suicidal behavior. We can only diagnose this on autopsy. Now, there is no screening test yet for this. Just recently big study in Jama out in July found evidence of cte in 99% of deceased NFL players' brains. Now there could be biased involved because those players may have been having estimate symptoms and donated their brains for study or other terps involved in terms of the pathology. In the meantime, it's Friday morning. "Friday night lights" and so many going to the games to watch their kids play and see stories and wonder about the risks. It's scary and concerning but you can't remove all risks. There are other sports. This is not just football that has a risk of head trauma. Recent study show children who play contact football before the age of 12 have three times higher risk of symptoms and depression. We need to understand how to prevent it, screen it and treat it. You know your kids best so watch the warning signs. Over to you.

This transcript has been automatically generated and may not be 100% accurate.

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