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

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.

It was not clear what caused their deaths but the DMRE said an investigation into the incident with the help of Harmony Gold – the previous owner of the mine which ceased operations in the 1990s – determined that methane levels were very high in the ventilation shaft.

Authorities in Free State were being prevented from sending a search team into the mine to retrieve bodies because there were still dangerously high levels of methane gas in the shaft, the department said in its statement on Friday.

It said it was considering various options to “speedily deal” with what was “a unique and strange situation”.

Illegal prospecting is rife in South Africa’s old gold-mining areas, where miners go into closed and often dangerous mine shafts to dig for deposits.

The government department said it had received information that three bodies had been recovered after they were brought to the surface by other illegal miners. Another 16 suspected illegal miners who were also in the shaft have handed themselves over to authorities, police said.

Sihle Maake, Harmony’s spokesperson, said the company viewed the incident – which occurred on May 18 – as a criminal matter and has handed it over to the South African police for further investigation.

“Harmony is working with the DMRE to provide direction on the next steps,” Maake said.