South African hotel and gaming group Sun International said on Tuesday that Nigeria'sEconomic and Financial Crimes Commission (EFCC) had launched a probe into its initial investment in the Tourist Company of Nigeria (TCN).

The investigation is the latest cloud over South African investment into the continent's largest economy after Nigerian authorities imposed a $3.9 billion fine on telecoms group MTN for failing to disconnect users with unregistered SIM cards.

Four Sun employees, three South Africans and one Nigerian, were detained in Nigeria last week and the company said in a statement that it had secured their release Monday night.

"The EFCC still wishes to investigate the nature of Sun International’s original investment into TCN as well as TCN’s trading records and has requested that TCN provide them with information and documentation," Sun said.

A Sun spokesman said the group's investment to date into TCN, which runs two hotels and a casino, was $50 million.