Police have confirmed an investigation is looking at whether the switching on of a conveyer belt caused the second explosion at the Pike River Mine.
The explosion - five days after the initial blast - led to the rescue operation for the 29 men trapped inside being called off.
In a statement, police said the theory was first raised in 2017 and they had received a number of requests for information about this.
"Police have already spoken to a number of current and former police staff involved in the operation, and are currently reviewing relevant documentation."
Police said there was no information to suggest the conveyor belt was activated on 24 November, prior to the second explosion, no documentation to suggest that police or anyone else authorised the conveyor belt being activated, and no information to suggest that others activated the conveyor belt without police knowledge.
Any decision to turn on the belt would have to have been signed off by the police who were in charge of the mine at the time.
Police said the investigation was ongoing.