The American football player Junior Seau, who committed suicide last year, had a debilitating brain disease, likely from 20 years of knocks to the head.
ABC News and ESPN have made the claim, citing a report from researchers and his family as sources.
The 20-year NFL veteran died in May after shooting himself in the chest.
A study of Seau's brain by a team of independent researchers found that he had suffered from chronic traumatic encephalopathy, or CTE, the report said.
CTE can only be diagnosed after death.
Tissue from his brain was sent to the National Institutes of Health for analysis in July at the request of Seau's family amid growing concerns over the long-term effects of football-related head injuries.