And now tonight, we have an abc news/espn exclusive report that's jolted the world of professional football. Junior seau was the star player who killed himself last may. And now abc's jim avila breaks... See More
And now tonight, we have an abc news/espn exclusive report that's jolted the world of professional football. Junior seau was the star player who killed himself last may. And now abc's jim avila breaks the news that he did have a brain disease linked t head trauma, like the kind he endured in football. Reporter: An nfl warrior, junior seau, who over a 20-year career made an estimated 1,8 a 0 bruising tackles, most of them head. First. He'd come out and jus hit the blackout button in the bedroom and black out the room so he could lay down. Reporter: And today in medical documents obtained by abc news and espn, and confirmed in an exclusive interview seau's family, we learned for the first time that this monster of the middle, who hlledimself last year, suffered severe brain damage from repeated blows to the head. We all love a good hit and a competitive game where they're battling, but at the end of the game, is it really worth it? Reporter: Seau's family donated his brain to science. The national institute of health found evidence of shrunken and hardened brain cells like these, tell-tale signs of chronic traumatic encephalopathy. All concurred with the diagnosis of cte. Reporter: It can be a lethal chain of events. Chronic blows to the head, including concussions, can cause cte. The symptoms include dementia, mood swings and depression severe enough to leave to suicide. The combination of depression and lack of impulse control probably contributes to the suicide that we see in this group of individuals. Reporter: Researchers at have documented 50 football players stricken by cte, 35 of them nfl players, six high school players. Cte is not being seen in individuals without repetitive head injury in our experience, Reporter: Today's revelation about the toughest among them has some in the nfl questioning their own health. The times I do forget things, the sleepness nights you may have, you start to wonder, are you down that path towards cte? Reporter: The seau family noticed his sudden changes in personality in the last years of his life. From outgoing hard-charging family leader to withdrawn loner who ended it all by shooting himself in the chest, leaving this three-word message on a text to his four children and ex-wife. And what did he say? Just three words. "I love you." That was the last we heard. Reporter: None of seau's children play football anymore and that's fine with their mother. I think it's a gamble. If you want to play the sport, know what you could possibly be in for. It's not worth it for me to not have a dad. So to me, it's not worth it. Reporter: The nfl said today it is financing more research and acknowledges it has more work to do and they promise to do it. Man of 1,000 hits now, diane, may change this sport forever. And this is a big development. I know you'll have more reporting tonight on "nightline," be sure to watch at its new time, 12:35 a.M. Eastern.
This transcript has been automatically generated and may not be 100% accurate.