Some families of Pike River mine victims suspect a piece of vital evidence may have been spirited away by the mining company and lost.
Sonja Rockhouse, whose son Ben was among the 29 men killed in the blast, said a photograph of an electrical cabinet door that was blown 100m to the mine surface has just come to light, but no-one can say where the actual item has gone.
The door from a fan control box was photographed nine days after the first explosion in 2010.
"It's a possible ignition source - it could be the thing that caused the explosion, so it's a vital piece of information," said Ms Rockhouse.
She said one of the "unsafe" fan control cabinets was blown to the surface and had disappeared onto a helicopter leased by the Pike River Mine company.
"It's bloody outrageous and it shows exactly why we need to go back into Pike and why we need a proper investigation."
Bernie Monk, whose son Michael also died, was angry the photograph had taken so long to come to light.
"Someone knew that incredibly important evidence had turned up and somebody knows what happened to it. Whoever that is needs to come forward right now, because we're not stopping until we uncover the truth."
Anna Osborne, whose husband Milton lost his life at Pike, said there were still many unanswered questions.
"All I can say is it's a good thing we fought so hard for re-entry and re-investigation, because with each day it's clear there's more that New Zealand needs to know," Ms Osborne said.
The former chief mines inspector, Tony Forster, who is currently back on the West Coast advising the Pike River Re-Entry project, said he first saw the photograph in 2013 or 2014, but was told it was a line of enquiry that had "gone cold".
It was not until he showed it to the families last week that he learned they had never been aware of it at all.
He said he tried to track down the object with no success and has since made a formal statement to the police.
Police Assistant Commissioner Tusha Penny said she became aware of the matter last week and police were making enquiries.
"Police would like to hear from anyone with information which could assist us on this, or any other issue which may be relevant to the original investigation," Ms Penny said.
"Police are absolutely committed to ensuring we provide answers to the Pike River families and we are working hard to provide all support necessary to the re-entry operation."
Pike River Coal went into receivership in December 2010.