At least 31 illegal miners are believed to have died in a gas explosion in a disused gold mine in South Africa that happened more than a month ago but was only now coming to light, authorities said Friday.

A search operation of the mine was being delayed because methane gas levels were still dangerously high in the ventilation shaft where the miners are thought to have died, the national Department of Mineral and Energy Resources said in a statement.

The mine in the city of Welkom in the central Free State province was previously operated by South Africa’s largest gold-mining company but had been shut down in the 1990s, the department said.

The department, which is the South African government ministry responsible for mining, said it was still piecing together the details of the incident, which it believes involved illegal miners from the neighboring country of Lesotho.

The miners are believed to have died in an explosion in Shaft 5 of the Virginia mine on May 18. Lesotho's foreign ministry recently provided some information to the South African authorities, the department said.

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Illegal prospecting is rife in South Africa’s old gold-mining areas, where miners go into closed and often dangerous shafts to dig for any deposits left behind. Fatal incidents involving illegal miners are common and sometimes go unreported because survivors are afraid of being arrested when they inform authorities.

In November, South African police discovered the bodies of 21 illegal miners at a mine in use in Krugersdorp, a town west of Johannesburg. Authorities said they believed the bodies had been moved to the active mine from a different disused mine by other illegal miners so they could be discovered.

Regarding the gas explosion last month, the mineral resources department said it had information that three bodies had been brought to the surface by other illegal miners but there were likely still dozens underground.

"It is currently too risky to dispatch a search team to the shaft," said the department. "However, we are considering various options to speedily deal with the situation."

The department called it "a unique and strange situation."

The mine was previously owned by Harmony Gold, according to the mineral resources department. Harmony’s chairman is billionaire mining magnate Patrice Motsepe, one of South Africa’s richest men and the brother-in-law of South African President Cyril Ramaphosa.