The white flash reported by Pike River Mine survivor Daniel Rockhouse as he staggered from the tunnel just after it exploded, helped convince experts the disaster may been caused when gases unleashed by a roof fall met sparking electrical equipment.
The Royal Commission of Inquiry resumed in Greymouth today, and finally delivered some answers about the deadly November 2010 explosion that killed 29 men underground.
The commission is leaning heavily on the Department of Labour's own investigation report, with a panel of five experts, to explain what happened on the afternoon of November 19, 2010.
Australian expert David Reece went through many different scenarios - an auxiliary fan was one possible ignition source, and something could have hit a pipeline.
But Mr Rockhouse, the survivor closest to the seat of the blast, saw a white flash, that indicated an electrical problem. It probably originated in a part of the known as Spaghetti Junction.
Chemical analysis and the size of the blast pointed to a gas - not a coaldust - explosion.
Commission chairman Justice Graham Panckhurst said the preferred expert theory was that a large collapse in the 'goaf', the void left behind by mining, had occurred.
This sent methane rushing through the mine, diluting as it went, until it reached the explosive range.
About the same time, the surface control room operator Daniel Duggan had turned on the water pumps into the mine, although not to the hydro monitor coal-cutting machine at the coalface.
This in turn re-powered the electrical system. The Department of Labour's experts believe this caused the variable speed drive - which had components throughout the mine and moderated the supply of current - to spark. Problems with variable speed drives have been reported in Australia.
There was also possible arcing.
"Nothing Daniel did was wrong,'' said commissioner David Henry.
Mr Duggan's brother Chris, also a coal miner, died in the blast.
The department has written to mining companies and Australian regulators expressing concerns about variable speed drives.
Under cross-examination, the department's lead investigator Brett Murray said they had relied heavily on circumstantial evidence and there were still a number of unknowns because no one had made it back underground to conduct a scene examination.
He told former Pike River Coal chief executive Peter Whittall's lawyer Stacey Shortall that former mine managers Mick Lerch and Kobus Louw had not been interviewed for the report.
The department had been unable to rule out that someone carrying contraband underground had caused the explosion.
Ms Shortall said "one piece of the puzzle'' was missing from the report, because the department had not looked at whether its own inspectors made mistakes.
Those inspectors came under fire at the commission last year for failing to spot ongoing safety issues at the mine.
Ms Shortall also said that after the disaster one inspector sat in on up to 18 official interviews with other mine employees.
Some equipment was not inspected until many months after the blast.
Mr Whittall, Valley Longwall Drilling and Pike River Coal Ltd (in receivership) have all been charged by the department in connection with the deaths.