Sept. 1, 2011 -- Three of the National Hockey League's hard-hitting enforcers have died since May, highlighting the emerging link between head trauma and mental illness.

Wade Belak, the 35-year old former forward for the Nashville Predators, hanged himself Wednesday in a Toronto condominium, the Associated Press reported. His death followed the fatal drug overdose of former New York Ranger Derek Boogaard, 28, in May, and the suicide of Winnipeg Jet Rick Rypien, 27, in August.

"It's not only about the deaths, it's the deaths that surround similar type players," Craig Button, former general manager of the Calgary Flames, told the news agency Canadian Press. Belak played for the Flames from 1998 to 2001. "It's not just getting hit in the head, it's everything that goes with that role. I think that people are paying very, very serious attention to concussions and blows to the head and the role of the enforcer."

An enforcer's role, albeit unofficially, is to fight. As such, Belak, Boogaard and Rypien were among the NHL most aggressive players, and arguably those most likely to take a punch.

Evidence to support the cumulative effects of repeated mild brain injuries is mounting. A recent study found that professional football players and boxers had chronic traumatic encephalopathy -- a progressive brain disease with that shares characteristics with Alzheimer's and Lou Gehrig's disease.

"I think for all of these things there's an individual susceptibility that's based on someone's genetic makeup as well as any potential injury they've had in the past," said Dr. Alan Hoffer, assistant professor of neurological surgery and neurocritical care at University Hospitals Case Medical Center in Cleveland. "It's not like every enforcer in the NHL has gone on to have this happen, the same way not every lineman in the NFL goes on to have dementia."

Enforcers, sometimes dubbed "goons," may have a heightened suicide risk because of the position's inherent impulsiveness.

"These things don't cluster together by accident," said Dr. Richard Shelton, professor of psychiatry at Vanderbilt University Medical Center. "Impulsiveness, impulsive acts of violence and impulsive suicide attempts seem to kind of ride along together."

In February, former Chicago Bears defensive back Dave Duerson died of a self-inflicted gunshot wound at age 50. He shot himself in the abdomen, requesting in a suicide note that his brain be studied for evidence of chronic traumatic encephalopathy. And in July, seven former National Football League players filed a class action arguing the league failed to properly treat concussions and tried to conceal the link between football and brain injuries.

"In the past, we didn't make the connection between brain injuries and things like depression," said Hoffer. "But now we're much more aware of how they're related."

For Belak, Boogaard and Rypien, hits to the head were routine. And while equipment improvements can help dissipate the force of flying fists, concussions are an inevitable consequence of contact sports.

"Obviously, people have the freedom to do what they want to do to their bodies," Hoffer said. "I think things start to get a little bit controversial when you're talking about children. Then there's much more of an argument that we should be doing more to protect them."

Last September Gilbert Allen Austin Trenum III, 17, of Prince William County, Va., hanged himself, an act his parents believe stemmed from the concussion he'd endured two days before in a high school football game. His parents, Gil and Michelle Trenum have donated his brain to research.

Belak played 549 career games in the NHL, earning eight goals, 25 assists and 1,263 penalty minutes. He fought 136 times.

The Associated Press contributed to this report.