The independence of an investigation into the second Pike River Mine explosion is being called into question.
It comes after video footage was released this week by the families of the 29 men killed, which they said showed the explosion was caused by a conveyer belt being switched on.
Police said while the idea was considered, they could find no evidence this was authorised.
However, they had now launched an investigation to ensure their actions did not cause the second explosion, which scuppered all hope of finding any of the men alive.
Families spokesperson, Bernie Monk, said police could not investigate themselves and an independent agency should instead be put in charge.
"I'm not saying the police are not professional at their job. But I cannot understand how the police now can investigate themselves on what they did up at Pike River."
Police have been approached for comment.