CTE has also been found in hockey players, wrestlers and boxers. It's still not possible to diagnose while a person is alive.
Seau's death in May prompted NFL player Jacob Bell to quit the sport altogether, leaving behind his contract with the Cincinnati Bengals.
"We're getting so much money, so much glory, so much fame; we're boosting our egos so much by playing a sport that's violent and could later on risk our lives," Bell said in May.
CTE researchers from Boston University were unavailable, and do not comment on players' deaths until more research is obtainable.
The Associated Press contributed to this report.