Thirty-one miners have died after a methane explosion at an abandoned gold mine in South Africa.

The miners, suspected to be illegally working on the project, were killed by the explosion on 18 May, but the deaths were only announced on Friday. The Virginia Mine’s shaft 5 has not been officially operated since the 1990s. The mine was abandoned three decades ago after a first methane explosion. It was acquired by Harmony in the 1980s but closed shortly after as a result.

The South African Minister for Mineral Resources and Energy, Gwede Mantashe, said in a tweet that he would be visiting the site alongside investigators from his department, in order to pursue an investigation and aid the retrieval of the bodies. Three bodies have already been retrieved by other alleged illegal miners.

The deaths occurred in South Africa’s Free State province on the border with Lesotho, which initially relayed the message of the deaths to South Africa’s Department of Mineral Resources and Energy (DMRE).

Before its closure the mine was operated by South African gold miner Harmony Gold. South Africa’s DMRE said in a statement: “Working in collaboration with the previous owners of the mine, Harmony, the DMRE inspectors have assessed the situation and determined that methane levels at the mine ventilation shaft 5 are very high. As such, it is currently too risky to dispatch a search team to the shaft.”

The deaths came just weeks after four miners died at a Sibanye-Stillwater gold mine in the neighbouring state of Mpumalanga. The workers at the Burnstone gold project died when the gantry they were standing on collapsed. This means that despite a fatality-free January for the first time ever in the country’s mining industry, mining deaths have surpassed more than half of 2022’s record low total.