McKee said in an interview that she was aware of a third former soccer player who had C.T.E. but that she was not yet authorized to publicly identify the person.

Image Bellini holding the 1958 World Cup trophy at an anniversary celebration in 2008. His death was first attributed to complications related to Alzheimer’s disease. Credit... Joedson Alves/Agence France-Presse — Getty Images

As C.T.E. began to gain widespread attention about six years ago, it was often thought of as an American problem. Many of the early cases of the disease, for which there is no known cure, were connected to boxers and American football players.

But more recently, evidence has mounted to indicate that those at risk for developing C.T.E. include soccer players. McKee said that although it was too early to say whether heading of balls was a cause of C.T.E. in soccer, it was becoming apparent that players were at risk of long-term brain trauma.

“I think there’s been a perception that the nonhelmeted sports are somehow less likely or less prone to these kinds of diseases,” she said. “There was also a time when people said C.T.E. was only an American problem. I think we are learning that, in both cases, those things aren’t true, and this is a problem that is going to be seen around the world.”

Dr. Lea T. Grinberg, a neuropathologist specializing in brain aging who has been affiliated with the University of São Paulo and is an assistant professor at the University of California, San Francisco, led the study of Bellini’s brain and presented her analysis recently at the International Congress of Neuropathology in Brazil.