The mystery around a missing piece of evidence could point to the cause of the Pike River mine disaster, according to a former chief mines inspector in the UK.
The door from a fan control box was photographed nine days after the first explosion in 2010.
Tony Forster, who is now advising the families affected by the Pike River disaster, has tried to track down the object with no success.
Watch John Campbell's exclusive interview with Mr Forster in the video above.
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