Photo: The Canadian Press. All rights reserved.

A aerial view shows the damage caused by a tailings pond breach near the town of Likely, B.C., on August, 5, 2014. 

The construction of a mine tailings pond on top of a sloped glacial lake weakened the foundation and was akin to loading a gun and then pulling the trigger, a report on the disaster says.

A government-ordered report released Friday said the spill that gushed 24 million cubic metres of mine silt and water into nearby lakes and rivers in B.C.'s Interior was caused by an inadequately designed dam that triggered the disaster.

The tailings dam at the Mount Polley mine burst without warning at 1 a.m. last August 4, hours after workers were at the foot of the structure.

Norbert Morgenstern, chairman of the panel that investigated the spill, said evidence indicates there was a glacial lake deposit under the foundation of the dam.

"The design did not take into account the complexity of the sub-glacial and pre-glacial geological environment associated with the perimeter embankment foundation," he told reporters gathered for the release of the report.

He said not taking that glacial lake into account and building on a weak layer was like loading a gun.

"But if constructing unknowingly on this upper (glacial) deposit constituted loading the gun, building with a steep slope ... pulled the trigger."

Morgenstern said the two factors together constituted the root cause of the failure.

The report also indicted that the failure was set off by construction of the downstream rockfill zone at the steep slope.

There was no indication that the dam was about to give way and no evidence that the failure was due to human intervention or so-called overtopping of the embankments, he said.

The gold and copper mine near Williams Lake remains closed.

Morgenstern is among a panel of three geotechnical experts appointed by the province a few weeks after the disaster to investigate the cause of the collapse and the role of government regulation and oversight.

Last month, B.C.'s chief inspector of mines allowed the owner of the mine, Imperial Metal Corp., to start repairs on the tailings pond.

The main concern for reconstruction was to ensure that increased water flow from melting snow this spring won't cause further environmental or human-health impacts, the chief inspector said.

A report released in November said the cleanup from the dam's breach will take many years to complete.

Environment Minister Mary Polak said then that the scale of the disaster was tremendous and that every effort was being made to clean up the mess.