B.C.'s worker safety agency ignored years of legal advice from the province's own lawyers before it bungled its probe of a 2012 mill explosion so badly that no one will ever face charges in the incident that killed two people, a report on the investigation says.

Premier Christy Clark said she is "deeply disappointed" with the errors and neglect revealed in the report written by one of her most senior deputy ministers on the WorkSafeBC investigation. Because of those errors, the Crown could not lay regulatory charges because most of the evidence gathered would be inadmissible in court.

WorkSafeBC's probe of the fire and explosion at the Babine Forest Products mill in Burns Lake was one of the largest in its history. The accident, believed to have been caused when fine sawdust from wood killed by pine beetles ignited in January, 2012, also injured 20 workers.

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The B.C. Ministry of Justice's prosecution service, the Criminal Justice Branch, announced last month that although WorkSafeBC had recommended four charges against the mill owners, none would be approved because Charter rights and due process had not been observed during the investigation.

The agency said last month that it followed protocols it had been using for years and that the Crown was changing the rules, but the report by John Dyble, deputy minister to the Premier, said that was not true.

Mr. Dyble found that WorkSafeBC should have known that, as far back as 2002, Supreme Court of Canada rulings had laid out that regulatory investigations – not just criminal ones – must be conducted in accordance with people's Charter rights.

Mr. Dyble found that the agency and the Criminal Justice Branch disagreed on the interpretation of the rulings and how they applied to occupational health and safety investigations.

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Mr. Dyble found the branch raised the Supreme Court rulings with WorkSafeBC in two other investigations between 2002 and 2013. As well, WorkSafeBC had been given training on the Supreme Court rulings, and admissibility issues "were reviewed on numerous occasions," Mr. Dyble wrote.

Two years before the explosion at the mill, the Criminal Justice Branch questioned whether WorkSafeBC had crossed the "bright line" set out by the Supreme Court in the probe of the deaths of three workers at a Langley mushroom farm in 2008.

Mr. Dyble found WorkSafeBC did not ask for legal advice from the branch on the Burns Lake investigation until after it was over.

Ms. Clark told reporters on Thursday she wants urgent action on the issues raised by the report, noting that WorkSafeBC is still conducting investigations, including one into a mill explosion in April, 2012, that killed two men in Prince George.

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"WorkSafe is doing investigations every single day, and we need to make sure all of them are meeting the investigative standard that's required," she said. "We need to be concerned about all of the serious investigations that are under way given what the report tells us."

Before her news conference, Ms. Clark summoned George Morfitt, chair of WorkSafeBC's board, to her office to demand the agency follow through on several of Mr. Dyble's recommendations. She would not comment on whether Mr. Morfitt should be fired.

"I am deeply disappointed with the way WorkSafe conducted this investigation," Ms. Clark said. "WorkSafe should have known what was required by the Criminal Justice Branch in order to proceed with a case to court."

The report recommends WorkSafeBC and the Crown negotiate a memorandum of understanding on issues of disclosure and recommendations to the Crown on charges, as well as working towards better communication during investigations.

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Leonard Doust, a prominent Vancouver lawyer, has been hired as a government adviser to see the recommendations are enacted.

The Premier said based on a legal opinion by Mr. Doust, there is no prospect of a reversal in the decision not to lay charges.

Ms. Clark also said she wants an explanation from WorkSafeBC on why the agency did not ensure it understood the rules of evidence required by the Criminal Justice Branch.

"WorkSafe must explain and examine why that happened," she said.

In a statement on its website, WorkSafeBC acknowledged the release of the "valuable" report and said it "will be working quickly" towards its implementation with information on how it will proceed to be provided soon.

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Harry Bains, the labour critic for the Opposition New Democrats, said an independent inquiry would be the best means of answering all questions about the issue so the NDP will keep pushing the government to consider that possibility.

"At least it would "clear the air," he said.

Lucy Campbell, whose brother Carl Charlie was killed in the blast, said she had hoped for a public inquiry and was disappointed by Thursday's developments.

"We're not even close to having closure," she said.