Authorities say a search is yet to be launched because of dangerously high levels of methane after May explosion.

The accident happened on May 18 in the city of Welkom but has only come to light, South African authorities said on Friday. The bodies are still underground.

Officials said it is likely there were more miners underground than initially thought and the death count could be higher than 31. A search operation has not yet been launched because of dangerously high levels of methane at the mine and the possibility of more explosions.

“As it stands now, it’s rather difficult,” Department of Mineral Resources and Energy spokesperson Nathi Shabangu said on Tuesday.

Shabangu said the government was working with a team of inspectors from Harmony Gold, the company that previously operated the mine, and mine rescue services to determine if the underground activity detected is human movement.

Fatal accidents involving people engaged in unlicensed mining often go unreported in South Africa because any survivors are reluctant to contact authorities because they might be arrested.

The incident at the Welkom mine was also complicated because the miners are from neighbouring Lesotho, and it took weeks for their families to report them missing to Lesotho authorities who then contacted their South African counterparts.

Miners working illegally do take food, water and other supplies with them because they expect to be underground for some time.

But given that the explosion was suspected to have happened in May, chances of there being survivors are minimal, and the mineral resources department could not confirm what the movements detected were until it had more information, Shabangu said.

A Harmony Gold spokesperson said on Monday that the methane levels in the mine meant that “we’re currently not allowing anybody to go there.” She said it was too dangerous right now for a search and rescue team to enter the mine.

Illegal gold mining is rife in South Africa, where people go into mines that are no longer commercially viable in the hope of striking it rich by finding deposits left behind. The illegal mining comes with high risks, and fatal accidents are fairly common.

In 2009, 82 miners mostly from Zimbabwe, Mozambique and Lesotho died after inhaling toxic gas following a fire in a disused shaft of a different gold mine in Welkom.

On Monday, South Africa’s minister of mineral resources and energy, Gwede Mantashe, visited Welkom and said it was likely that there were more than 31 deaths in the latest incident.

Mantashe said initial investigations showed that last month’s explosion in the mine, which was shut down in the 1990s, had caused collapses that had sealed off access to the shaft where the miners were. That and the presence of methane meant a search operation would take “a bit longer”, he said.

“If it takes longer, it takes longer,” Mantashe said. “But we must take those bodies out.”