Researchers from the National Institutes of Health said Thursday the former NFL star's abnormalities are consistent with chronic traumatic encephalopathy, or CTE.
The hard-hitting linebacker played for 20 NFL seasons with San Diego, Miami and New England before retiring in 2009. He died at age 43 of a self-inflicted gunshot in May, and his family requested the analysis of his brain.
"We saw changes in his behavior and things that didn't add up with him," his ex-wife, Gina, told The Associated Press. "But (CTE) was not something we considered or even were aware of. But pretty immediately (after the suicide) doctors were trying to get their hands on Junior's brain to examine it."
The NIH, based in Bethesda, Md., studied three unidentified brains, one of which was Seau's, and said the findings on Seau were similar to autopsies of people "with exposure to repetitive head injuries."
"It was important to us to get to the bottom of this, the truth," Gina Seau added, "and now that it has been conclusively determined from every expert that he had obviously had CTE, we just hope it is taken more seriously. You can't deny it exists, and it is hard to deny there is a link between head trauma and CTE. There's such strong evidence correlating head trauma and collisions and CTE."
In the final years of his life, Seau had wild behavioral swings, according to Gina and to 23-year-old son, Tyler, along with signs of irrationality, forgetfulness, insomnia and depression.
"He emotionally detached himself and would kind of 'go away' for a little bit," Tyler Seau said. "And then the depression and things like that. It started to progressively get worse."
He hid it well in public, they said, but not when he was with family or close friends.
Seau joins a list of several dozen football players who were found to have CTE. Boston University's center for study of the disease reported last month that 34 former pro players and nine who played only college football suffered from CTE.
The NFL faces lawsuits by thousands of former players who say the league withheld information on the harmful effects of concussions. According to an AP review of 175 lawsuits, 3,818 players have sued. At least 26 Hall of Famer members are among the players who have done so.
The National Football League, in an email to the AP, said: "We appreciate the Seau family's cooperation with the National Institutes of Health. The finding underscores the recognized need for additional research to accelerate a fuller understanding of CTE.
"The NFL, both directly and in partnership with the NIH, Centers for Disease Control and other leading organizations, is committed to supporting a wide range of independent medical and scientific research that will both address CTE and promote the long-term health and safety of athletes at all levels."
NFL teams have given a $30 million research grant to the NIH.
The players' union called the NIH report on Seau "tragic."
"The only way we can improve the safety of players, restore the confidence of our fans and secure the future of our game is to insist on the same quality of medical care, informed consent and ethical standards that we expect for ourselves and for our family members," the NFLPA said in a statement.
"This is why the players have asked for things like independent sideline concussion experts, the certification and credentialing of all professional football medical staff and a fairer workers compensation system in professional football," it said.
Seau is not the first former NFL player who killed himself and later was found to have had CTE. Dave Duerson and Ray Easterling are the others.
Before shooting himself, Duerson, a former Chicago Bears defensive back, left a note asking that his brain be studied for signs of trauma. His family filed a wrongful-death suit against the NFL, claiming the league didn't do enough to prevent or treat the concussions that severely damaged his brain.
Easterling played safety for the Falcons in the 1970s. After his career, he suffered from dementia, depression and insomnia, according to his wife, Mary Ann. He committed suicide last April.
Mary Ann Easterling is among the plaintiffs who have sued the NFL.
Tyler Seau played football through high school and for two years in college. He says he has no symptoms of brain trauma.
"I was not surprised after learning a little about CTE that he had it," Tyler said. "He did play so many years at that level. I was more just kind of angry I didn't do something more and have the awareness to help him more, and now it is too late."
Gina Seau's son Jake, now a high school junior, played football for two seasons but has switched to lacrosse and has been recruited to play at Duke.
"Lacrosse is really his sport and what he is passionate about," she said. "He is a good football player and probably could continue. But especially now watching what his dad went through, he says, 'Why would I risk lacrosse for football?'
"I didn't have to have a discussion with him after we saw what Junior went through."
Her 12-year-old son Hunter has shown no interest in playing football.
"That's fine with me," she said.