Former Wild enforcer Derek Boogaard had a degenerative brain disease linked to repeated head trauma when he died in May at age 28.

The disease, chronic traumatic encephalopathy (CTE) is a close relative of Alzheimer's disease and has been diagnosed in the brains of more than 20 former football players. It can be diagnosed only posthumously.

The researchers at the Boston University Center for the Study of Traumatic Encephalopathy who examined Boogaard's brain said the case was particularly sobering because Boogaard was a young, high-profile athlete, dead in midcareer, with a surprisingly advanced degree of brain damage.

"To see this amount? That's a 'wow' moment," said Ann McKee, a neuropathologist and a co-director of the center.

Boogaard played five seasons for the Wild before signing with the New York Rangers in 2010. He suffered a concussion and was sidelined most of that season. On May 13, his brothers found him dead of an accidental overdose in his Minneapolis apartment.

The degenerative disease has been found in the brains of all four former NHL players examined by the Boston University researchers. The others were Bob Probert, who died at age 45; Reggie Fleming, 73; and Rick Martin, 59.

CTE is believed to be caused by repeated blows to the head. Scientists say it shows itself in symptoms like memory loss, impulsiveness, mood swings, even addiction.

Damage was spreading

Scientists told Boogaard's family they were shocked to see so much damage in someone so young. It appeared to be spreading through his brain and, had he lived, his condition likely would have worsened into middle-age dementia.

Boogaard was not in the Wild lineup at the start of the 2009-10 season because he was being treated at the Canyon Center in Malibu, Calif., for an addiction to prescription drugs. The Wild, at the time, said he was suffering from a concussion because Boogaard was worried the revelation would harm his reputation.

When he returned, he received at least 11 different prescriptions for painkillers from eight doctors, his father, Len Boogaard, told the Times. The Times also reported Derek Boogaard bought painkillers from acquaintances. Boogaard played in 57 games and did not score a goal for the Wild. Following the season, he signed as a free agent with the Rangers for four years and $6.5 million.

"It's one of the great cities to be at and you're always on center stage when you're out there, so I'm excited," Boogaard told the Star Tribune the night he signed with the Rangers.

Boogaard, a fan favorite affectionately dubbed the "Boogey Man" during his days policing the ice at Xcel Energy Center, was not signed for his goal-scoring prowess.

Fun-loving and gentle off the ice, standing 6-foot-8, he was also one of the NHL's most feared and prodigious fighters.

In the offseason, he and his brother ran a camp teaching hockey brawling techniques.

The Rangers knew about Boogaard's substance-abuse problem and time in rehabilitation, family members told the Times. While playing for the Rangers, Boogaard was injured during a fight with Ottawa's Matt Carkner in December, 2010, hitting his head on the ice. He suffered a severe concussion, and was unable to leave his apartment for weeks. According to a former teammate, Devin Wilson, Boogaard bought painkillers from a source in Long Island.

He did not play again during the season and returned to the Canyon Center for more addiction treatment in April. He flew to Minneapolis the day of his release, and died after a night of heavy partying with his brother, Aaron.

McKee has examined nearly 80 brains of former athletes, mainly retired football players and boxers who spent their careers absorbing blows to the head. The center's peer-reviewed findings of CTE have been widely accepted by experts in the field. The NFL, initially dismissive, has since donated money to help underwrite the research.

NHL sees no link

The NHL is not convinced that there is a link between hockey and CTE.

"There isn't a lot of data, and the experts who we talked to, who consult with us, think that it's way premature to be drawing any conclusions at this point," NHL Commissioner Gary Bettman told the Times. "Because we're not sure that any, based on the data we have available, is valid."

The researchers at Boston University say that CTE is a nascent field of study, but that there is little debate that the disease is caused by repeated blows to the head. They said that the NHL was not taking the research seriously.

"We don't know why one person gets it more severely than another person, why one person has a course that is more quick than another person," said Dr. Robert A. Stern, a neuropsychologist and a co-director for the Center for the Study of Traumatic Encephalopathy. "But what we are pretty sure of is, once the disease starts, it continues to progress."

Linking CTE to Boogaard's rapid descent in his final years is complicated by his drug addiction.

"He had problems with abuse the last couple years of his life, and that coincided with some of the cognitive and behavioral and mood changes," Stern said. "What's the chicken? What's the egg?"

Officials from both the Wild and the Rangers declined comment to the Times.