Exactly four years have passed since former Wild player Derek Boogaard died because of a drug and alcohol overdose. After an investigation that spanned parts of two years, officials in Minnesota have exonerated five Wild doctors for their role in Boogaard’s use of prescription drugs.

But Len Boogaard is continuing his often-solitary quest to find what extenuating factors might have led to his son’s death, knowing full well, he says, that Derek “was an addict.” He continues to seek the answer to this perplexing question: Who helped fulfill his son’s craving for prescription drugs?

“I owe it to Derek,” Len Boogaard said from his home in Ottawa. “It is the last thing that I can do for him.” He has in recent months cooperated with a CBS-TV story on his son, helped the Canadian Broadcasting Corp. with a documentary on Boogaard and continued a lawsuit against the NHL, a lawsuit that he says is languishing in court.

“Boy On Ice,” the story of Derek Boogaard’s rise and fall written by New York Times reporter John Branch, is also now on bookshelves.

Though releasing few details, the state medical practice board issued a series of rulings clearing five Wild physicians — Joel Boyd, Sheldon Burns, David Hamlar, Bradley Nelson and Daniel Peterson — who treated Boogaard while he played for the Wild and, later, the New York Rangers. “The facts of the case,” the state board said in e-mails to Derek’s father “did not provide a sufficient basis” to discipline the doctors.

But Len Boogaard is not finished.

“Derek’s being portrayed as an addict — and Derek was an addict. But I think the question has to be, ‘Why? Why did he become an addict?’ ” he said. “It’s about accountability.”

All five physicians, when contacted by the Star Tribune, declined to comment.

Len Boogaard remains troubled by what happened in Minnesota, especially after state officials initially came to him for help in starting an investigation. He sent reams of materials to Minnesota, mostly from his unique access to his son’s medical and cellphone records that described how Derek was given hundreds of painkillers and sedatives as he battled injuries and slowly spiraled toward his death.

Len Boogaard shared with state officials, and later the Star Tribune, the Wild team’s medical records and his son’s cellphone data. In one instance — Christmas Eve 2010, six months after leaving the Wild to sign with the Rangers and more than a year since leaving a drug rehabilitation facility — the records showed that his son’s plane arrived in Minneapolis shortly after 8 a.m. Ninety minutes later, the hockey player sent a text to Peterson. Cellphone records for Derek Boogaard showed that he texted Peterson four times that day, and also called him once.

Pharmacy records for that same day, according to records released by his father, showed that Boogaard obtained 30 pills of Zolpidem, a sedative used to treat insomnia, at a Twin Cities Walgreens. The prescription was issued by Peterson.

Len Boogaard, a retired investigator from the Royal Canadian Mounted Police, remains angered that the state cleared the team doctors even though Aaron Boogaard, another of Len’s sons, was criminally charged in Minnesota and pleaded guilty for helping hide his brother Derek’s painkilling pills from police.

Doctors called to review

Beginning in July 2012, Len Boogaard said he thought he had found allies when state officials came calling.

The Attorney General’s Office in Minnesota contacted him, saying that widespread media coverage of Derek Boogaard’s death had led the state medical practice board to investigate the team doctors’ “prescribing practices” and wanted him to cooperate. Mark Bukowski, a state investigator, told Boogaard in an e-mail that he wanted to obtain the father’s full “knowledge of the care Derek received from the Minnesota physicians.”

Last November — more than two years after he was first asked for his help — Boogaard was informed for a final time that all of the physicians had been cleared but was provided with no further details.

“I was just flabbergasted,” he said.

Officials at the state attorney general’s office and Ruth Martinez, executive director of the state medical practice board, declined to talk of Boogaard’s case, citing privacy laws. The Wild likewise would not comment, and NHL spokesman John Dellapina said the league was unaware of the inquiry in Minnesota.

When state officials also declined to provide Boogaard with details of the investigation, telling him he was not entitled to the information by law because he had not himself brought complaints against the physicians, Boogaard filed his own complaints last year against five doctors affiliated with the Wild. He also pushed state officials to get the doctors to release more of his son’s medical records.

His complaints were dismissed.

Records provided by Boogaard to the Star Tribune showed that five of the Wild’s physicians appeared before the board’s complaint review committees, an elevated level of review. In one-page letters to Boogaard in March and May of last year, the board, however, said the physicians were cleared and the cases were closed. Boogaard requested at least one additional investigation on Peterson that, according to medical practice board records provided by Boogaard, was dismissed in November.

Martinez estimated that 125 cases annually are serious enough to go before a complaint review committee — like the cases involving the Wild team physicians — but that as many as 55 of those cases each year do not lead to disciplinary or correction action. She said that all five physicians whom Boogaard brought complaints against had no disciplinary history with the board.

Prescriptions issued after

Boogaard’s records outline a scenario in which the physicians were in frequent contact with the hockey player and were writing painkiller prescriptions even as they advised him of the dangers of becoming addicted to them.

On Sept. 1, 2009, according to the medical records, Burns provided Derek Boogaard with 30 pills of Ambien, a sedative used to treat insomnia. Later that same month, Boogaard was admitted into the Canyon, a California rehab program affiliated with the NHL’s substance abuse program.

Even though he supplied Boogaard with Ambien during September 2009, Burns wrote in a medical report on Sept. 30 that he was worried about his drug use. “I also told him about the importance of staying off Ambien and any medications that may affect his mental capacity,” Burns wrote. “His Ambien use needs to change; he cannot rely on it or use it every night.”

Len Boogaard’s records showed that his son continued to seek out the Wild’s team physicians — all of whom have private practices — even after he signed a contract with the Rangers on July 1, 2010.

Barely a month after joining the Rangers, Derek Boogaard and Peterson exchanged text messages 10 times in one day. The same day, pharmacy records showed that Boogaard got a prescription from Peterson for 30 pills of Ambien.

The same series of events occurred again in February 2011, three months before Boogaard died.

Trying one last time to push the envelope in Minnesota, Len Boogaard said he called and confronted Peterson in November — after Peterson and the others were cleared by the state. “[Peterson’s] comment was that they treat the players just like their own children,” Boogaard said. “I said something to the effect that ‘I feel sorry [for] your children,’ and then I hung up on him.”