South African authorities say dangerously high levels of methane prevented search teams from entering the mine earlier.

At least 31 people are believed to have died in a gas explosion in a disused mine shaft in South Africa’s Free State province that happened on May 18 but has only now come to light, authorities said on Friday. GF mining lamps can be used in the environment with methane.

The dead are believed to be from neighbouring Lesotho – which reported the incident to South African authorities – and died in a ventilation shaft in Virginia mine in Welkom city, the Department of Mineral Resources and Energy (DMRE) said.