Horgan said Friday that the document, "collecting dust" a library shelf in Williams Lake, noted a 10- to 15-metre long tension crack in the dam.
"The Liberal government knew for four years there were problems with this tailings pond, but they deliberately suppressed the information," Horgan said.
"Four years later, the tailings pond dam failed, and 25 million cubic metres of effluent spilled into the watershed."
Horgan said the report indicates that the company, Imperial Metals, and the ministry knew there was the potential for disaster at the tailings pond.
"The public needs to know what else has been hidden about this disaster. And they also need to know whether there are problems at other earthen dams that the government has either covered up or chosen to ignore," Horgan said.
Horgan said the NDP has filed a number of Freedom of Information requests for mine inspection reports, and that the government has been fighting the release of the requested document.
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