Dr. Jeffrey Kutcher, a neurologist who heads the University of Michigan NeuroSport program and is an N.H.L. Players’ Association consultant to the concussion working group, said the elimination of fighting is a policy decision for the league and the players union to make, in much the same way cracking down on checks to the head was.

“In essence, I would say there’s no more evidence that fighting is bad for the brain than there is that hits to the head are bad for the brain. The amount of evidence is the same — essentially, very little. Yet the decision was made on a policy level, let’s take head shots out of the game. There’s no more evidence, or less, for head shots than there is for fighting.”

Researchers generally agree that there is a link between repeated blows to the head and C.T.E. But they are uncertain which specific hits lead to the disease, and whether some people have a genetic predisposition. In Boogaard’s case, the Boston University researchers noted that it was impossible to know whether the condition was caused by blows he sustained in fights.

Dr. Robert Cantu, a co-director of the Boston group and a prominent neurosurgeon in the area of head trauma in sports, said the evidence was strong enough to say that league officials “are putting people at risk by allowing fighting.” Dr. Robert A. Stern, a neurosurgeon and another co-director for the Boston group, said that the elimination of fighting was a policy matter for the N.H.L. to decide and that “we need to not overreact or make knee-jerk policy or rule changes.” But, he added, “If there’s a logical explanation for a larger number of hits and you can then reduce that number of hits, that makes sense to do.”

When asked if, as a neuropsychologist, he believed fighting should be allowed in hockey, Echemendia said: “My role is to provide the league with information regarding mechanisms of injury in concussion and the evaluation and management of the injury in the N.H.L. It is up to the league to analyze the information we provide along with other sources of information in making policy decisions.” 

Dr. Willem Meeuwisse, a University of Calgary epidemiologist and medical consultant to the N.H.L., said the goal of the concussion working group was “to try and look at scientific evidence for things we can do to prevent concussion, and if there isn’t evidence, then it’s hard for that group to make a firm recommendation.”

Until the cause of C.T.E. is better understood, Echemendia said, the league should not jump to conclusions about the damage caused by fighting.

“I think that there are some real and interesting findings related to C.T.E., that there’s a lot of important work that’s being done,” said Echemendia, referring to the Boston group’s findings of C.T.E. in the brains of all four former hockey players it has examined.

“But now that work is in its infancy, and it is very premature to draw any conclusions of any causal links between fighting and C.T.E. There’s just not enough evidence to support that.”

Dr. Michael J. Stuart, an orthopedic surgeon at the Mayo Clinic in Rochester, Minn., and chief medical officer for USA Hockey, acknowledged the range of opinions among medical experts.

“These are opinions, very educated opinions and very experienced people providing their opinions,” said Stuart, who was a chairman of last year’s Ice Hockey Summit: Action on Concussion at Mayo.

Stuart said he applauded the efforts of the N.H.L. to reduce the number of checks to the head. But “maybe they’re not ready to eliminate fighting now — I don’t know, but I certainly would hope that we head toward that to minimize risk,” he said.

“If we truly want to make every effort to reduce the risk of concussion in the sport of ice hockey we should eliminate fighting,” Stuart said. “There’s no doubt about that.”

The N.H.L. finds itself in a position somewhat similar to that of the N.F.L. several years ago, when researchers began diagnosing C.T.E. in the brains of former professional football players. N.F.L. officials and the league’s medical experts initially dismissed the findings. But as more cases surfaced and Congressional inquiries were conducted, the league was forced to make rules changes.

Dr. Michael Cusimano, a neurosurgeon at Toronto St. Michael’s Hospital who is involved in efforts to reduce sports injuries, criticized the N.H.L. for claiming it needed more data.

“We heard this about 40 years ago with cigarette smoking,” Cusimano said. “Sure, there can be more evidence, but there’s some evidence out there that fighting is clearly a cause of brain injuries.

“What’s the threshold of evidence that Gary Bettman needs to see this?”