An expert in mine safety says the spotlight has been thrown back on the ventilation fan as one of the most likely sources of the sparks that caused the initial explosion at the Pike River mine.
The Pike River Royal Commission has finished hearing evidence and is now working on its recommendations.
During the hearings, a device regulating the mine's power supply was blamed for contributing to the explosion.
On Thursday the makers of the device, Rockwell Automation, rejected that evidence as simplistic and outdated.
A coalmine safety adviser, David Feickert, says questions remain about the role played by the ventilation fan.
Mr Feickert says it was very unusual for such a fan to be located underground.