The Nigerian government on Tuesday said a whistleblower prompted an investigation into alleged infractions committed by telecoms group, MTN.
Reuters reports that an official of the Office of Nigeria's attorney general made the disclosure Tuesday.
Ladidi Mohammed, head of asset recovery, told Reuters in a phone interview that an investigation was carried out by the central bank after the whistleblower's tip was received.
The Nigerian government last week ordered the telecoms group to repatriate $8.1 billion that the Central Bank of Nigeria (CBN) said was illegally sent abroad.
The fine comes two years after MTN, Africa's biggest telecoms company, agreed to pay more than $1 billion to end a dispute in Nigeria over unregistered SIM cards.
The CBN said MTN repatriated the money based on illegally issued Certificates of Capital Importation (CCIs) with the 'connivance' of Standard Chartered Bank, Stanbic IBTC, Citibank and Diamond Bank between 2007 and 2015.
The apex bank said the affected banks would refund various amounts totaling N5.87 billion.
Standard Chartered was asked to refund N2.5 billion; Stanbic IBTC (N1.9 billion); Citibank (N1.3 billion and Diamond Bank (N250 million).
The banks were accused of committing "flagrant violation of extant laws and regulations of the Federal Republic of Nigeria, including the Foreign Exchange (Monitoring and Miscellaneous Provisions) Act, 1995 and the Foreign Exchange Manual, 2006."
But a defiant telecom firm, in a reaction to the sanctions, said it will vigorously defend its position on the issue, describing the sanction as regrettable.
The South African telecoms giant in a statement issued on Tuesday said all dividend repatriation out of Nigeria was done on the basis of equity capital and with valid certificates.