Editor’s note: Over six months, The New York Times examined the life and death of the professional hockey player Derek Boogaard, who rose to fame as one of the sport’s most feared fighters before dying at age 28 on May 13. This article, the third in a three-part series, recounts how the toughest man in the NHL met a tragic fate and examines some of the questions that remain unanswered about the role hockey played in his death.

Through the night and into the next day, as the scrolls across the bottom of television screens spread the news of Derek Boogaard’s death in May, the calls of condolences came, one after another.

Among them was a call from a stranger, first to Joanne Boogaard in Regina, Saskatchewan, then to Len Boogaard in Ottawa.

It was a researcher asking for the brain of their son.

An examination of the brain could unlock answers to Boogaard’s life and death. It could save other lives. The Boogaard family waited for results. One month. Two. Three. Two other NHL enforcers died, reportedly suicides, stoking a debate about the toll of their role in hockey.

Four months. Five.

The news came in a conference call to the family in October.

Boogaard had chronic traumatic encephalopathy, commonly known as CTE, a close relative of Alzheimer’s disease. It is believed to be caused by repeated blows to the head. It can be diagnosed only posthumously, but scientists say it shows itself in symptoms like memory loss, impulsiveness, mood swings, even addiction.

More than 20 dead former National Football League players and many boxers have had CTE diagnosed.

But this was different. The others were not in their 20s, not in the prime of their careers.

The scientists on the far end of the conference call told the Boogaard family that they were shocked to see so much damage in someone so young. It appeared to be spreading through his brain. Had Derek Boogaard lived, they said, his condition likely would have worsened into middle-age dementia.

And that was when Len Boogaard’s own mind went numb.

Rehab, pills and a new team

The Minnesota Wild prepared for the start of the 2009-10 season. Derek Boogaard watched from a distance.

The team said that Boogaard, the pre-eminent enforcer in the National Hockey League and a hugely popular Wild player, was sitting out a few weeks because of a concussion. Instead, he was at the Canyon treatment center in Malibu, Calif., being treated for addiction to prescription drugs.

Boogaard was embarrassed and worried that news of his addiction would shatter his reputation. He was also concerned that someone would take his role. From rehabilitation, he tracked the preseason fights of teammates and texted friends to gauge how badly he was missed.

Most NHL teams have about 10 affiliated doctors — specialists and dentists with practices of their own. Boogaard had learned that there was no system to track who was prescribing what.

In one three-month stretch of the 2008-09 season with the Wild, Boogaard received at least 11 prescriptions for painkillers from eight doctors — including at least one doctor for a different team, according to records gathered by his father, Len Boogaard.

Derek Boogaard increasingly wanted more pills. He became adept at getting them.

In downtown Minneapolis, Boogaard’s favorite hangout was Sneaky Pete’s, a sports bar that becomes a raucous club on weekend nights. And Boogaard often bought painkillers, thousands of dollars’ worth at a time, from someone he knew there, according to Boogaard’s brother Aaron.

Wild coaches saw the decline for a couple of seasons. Boogaard’s admirable work ethic had faded, and no one could pinpoint why.

“I just said to him one day: ‘What’s up? What’s up with you? Where is the guy I know?’ ” said Matt Shaw, who coached Boogaard as an assistant with the minor league Houston Aeros and, later, with the Wild. “Because he was not himself. And he didn’t have an answer. He didn’t want to look me in the eye.”

Boogaard had been drafted by the Wild in 2001, a seventh-round pick given little chance of making the NHL. The Wild shepherded him through three seasons in the minor leagues and molded him into the most fearsome player in hockey.

But by the 2009-10 season, Boogaard was 27, and his body carried a lot of mileage. He missed the start of the season while in rehabilitation, and his contract was to expire at season’s end.

There were plenty of other suitors. The New York Rangers and the Edmonton Oilers each offered four-year contracts paying more than $1.5 million a season.

He chose New York. He signed a four-year, $6.5 million contract — a rather ordinary salary among his new Rangers teammates, but striking among the fraternity of enforcers who typically play only a few minutes a game.

Manic, sullen and lonely

Boogaard had played 21 games for the Rangers when he took the ice in Ottawa on Dec. 9, 2010. After leveling an opponent with a legal check, Boogaard was chased by Matt Carkner, a 30-year-old enforcer who had spent most of his career patrolling the minor leagues.

The two bickered as they glided across center ice. They barely stopped before Carkner cracked Boogaard’s face with a right hand.

Boogaard usually responded to such shots with an angry flurry. This time, he turned his head away and held on to Carkner. He did not throw another punch.

“I noticed he kind of stopped fighting and I took him down and landed on top,” Carkner told reporters. “Obviously, if you land a punch on a guy like that it feels good. It feels good to take down a big man like that.”

The Rangers said Boogaard was out indefinitely with a shoulder injury. Ten days later, they revealed he was having headaches.

When Len Boogaard arrived in New York from Ottawa in January, he barely recognized his son. Several times over several days, the toughest man in hockey bawled in his arms.

“I had to hold him,” Len Boogaard said of Derek. “It was like when he was younger, when he was a little kid growing up. He just sobbed away uncontrollably.”

For weeks, Boogaard mostly shuttered himself inside his $7,000-a-month apartment in Manhattan. The Rangers told him to avoid the rink because the commute, the movement, even watching hockey could bring nausea. The team delivered a healthy meal to his door every afternoon, but Boogaard usually threw it away. His kitchen counter overflowed with fast-food packages.

Those who saw him in New York noticed his memory lapses were growing worse. Boogaard joked about them, saying he had been hit on the head too many times. But they also came to worry about his darkening personality and impulsive behavior. His characteristic sweetness and easy manner, his endearing eagerness to please, had evaporated.

By March, Boogaard resumed light workouts with the Rangers, whose doctors continued to supply him with prescription drugs. Mark Messier, the team’s Stanley Cup hero in 1994 and now a team executive, tried to motivate him with a pep talk.

A day or two later, a noodle-legged Boogaard fell during on-ice workouts. The Rangers recognized the symptoms.

It was early April, the last week of the regular season, and Boogaard was on his way back to drug rehabilitation in California.

“His chest wasn’t moving”

The night of May 12 began with a painkiller, a 30-milligram Percocet that Aaron Boogaard later told the police he handed his brother at their two-bedroom apartment in Minneapolis. Derek, hours out of rehabilitation, was bent on a party.

He had dinner with friends at a steak-and-sushi place, where he drank Jack Daniel’s and Cokes. The group shuffled among Sneaky Pete’s and three other downtown Minneapolis bars. At some point, or several points, Derek fueled the buzz with more prescription painkillers.

Once home in his second-floor apartment on North First Street, he spent time in the bathroom. He went to his bedroom at the end of the hall.

The next day, Aaron and another brother, Ryan, arrived and stepped into the back bedroom, expecting to find their older brother sleeping off a hangover. It was about 6 p.m. on Friday, May 13.

“I looked and it didn’t look right,” Ryan said. “Like, his chest wasn’t moving.”

The Hennepin County medical examiner ruled it an accidental overdose of alcohol and oxycodone, the active ingredient in painkillers like OxyContin and Percocet.

“The coroner said with that mixture, he probably died as soon as he closed his eyes,” Aaron said.

Researchers’ “Wow” moment

It did not take long for Dr. Ann McKee to see the telltale brown spots near the outer surface of Boogaard’s brain — the road signs of CTE. She did not know much about Boogaard other than that he was a 28-year-old hockey player. And the damage was obvious.

“That surprised me,” she said.

A neuropathologist, McKee is one of four co-directors of Boston University’s Center for the Study of Traumatic Encephalopathy and the director of the center’s brain bank. She 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 group may now have its most sobering case: 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,” McKee said as she pointed to magnified images of Boogaard’s brain tissue. “This is all going bad.”

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 said. “Because we’re not sure that any, based on the data we have available, is valid.”

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

For years, the NHL has tiptoed between the allure of its fast-paced, hard-hitting action and the need to protect star players. Several star players in recent years have been forced to retire early because of postconcussion symptoms.

But the league has shown little interest in ending on-ice fighting. The message is decidedly mixed: outlaw an elbow to the head during play, but allow two combatants to stop the game and try to knock each other out with bare-knuckle punches to the head.

“If you polled our fans, probably more would say they think it’s a part of the game and should be retained,” Bettman said.

Chris Nowinski, a former Harvard football player and professional wrestler who is another co-director of the Boston University center, is the one who usually makes the initial call to a grieving family to request the brain. In October, Nowinski attended a Bruins game in Boston. There was a fight, and he watched quietly as thousands of people stood and cheered while the players fought.

“They are trading money for brain cells,” he said.

A father hunts for answers

Len Boogaard, a cop and father, tries to make sense of it all. On leave from his desk job in Ottawa — a back injury years ago forced him off the streets — he patches together the remains of Derek’s world.

Like a detective, he dials contacts in Derek’s phone to ask who knows what. He explores hundreds of pages of phone records to reconstruct Derek’s relationships, his moods, his sleep patterns. He follows paper trails, trying to link the history of his son’s prescriptions to vague diagnoses in team medical reports.

Since the day of the funeral in May, Len Boogaard said, he has not heard from the Rangers.

The team refused to answer a detailed list of questions regarding their medical treatment of Boogaard during the season and his time in rehabilitation.

It also refused requests to speak to general manager Glen Sather and the team physician, Dr. Andrew Feldman, among others, about Boogaard. Instead, it emailed a four-sentence statement from Sather that read, in part, “We worked very closely with Derek on and off the ice to provide him with the very best possible care.”

For those who knew Derek Boogaard, there are questions that may never be resolved and regret that may never be relieved.

In July, Aaron Boogaard was charged with a felony for distributing a controlled substance — in this case, one pill to his brother on the night of his death. The charge was dropped in October. He also told the police that he flushed pills down the toilet after placing the 911 call. He pleaded guilty to tampering with the scene of a death and received probation and 80 hours of community service.

He is trying to revive his own hockey career, and with his name has come the expectation to fight. Now 25, he plays for the CHL’s Rio Grande Valley Killer Bees in Hidalgo, Texas. He wears No. 82, marking the year Derek was born. Aaron fought six times in a recent 10-game stretch.

His mother has asked him to quit hockey. But he has no Plan B, either.

“I mean, honestly,” Aaron said, “what else am I going to do?”