Research has shown that athletes such as footballers, rugby players, and fighters such as boxers have a significantly higher risk of developing CTE than the general population.

CTE can only be diagnosed after death. The disease, caused by repeated blows to the head, has been found in the brains of dozens of former National Football League (NFL) players.

In the early stage of the disease, some people also experience short-term memory problems, depression, aggressive tendencies and executive function problems, according to the CTE Society, a nonprofit group.

In later stages, these problems become stronger and others arise, such as mood swings and suicidal thoughts. In its final stage, a person may experience severe cognitive problems and memory loss, as well as dementia, according to the CTE Society.

The Boston University CTE Center said the cause of suicide is complex and a postmortem diagnosis of CTE is not known to be a risk factor for suicide.

But CTE researchers said the case showed it had not gone down enough to reduce the risk of the disease in athletes.

“Mr. Kneeland played in the modern era of concussion protocols and better helmets, and still developed CTE,” said Dr. Chris Nowinski, co-founder and CEO of the Concussion & CTE Foundation. “We have no reason to believe that the current generation has a lower risk of CTE than previous generations.”