A cyber attack on Mexicos interbank payment system allowed hackers to make off with more than $15 million in the past several weeks, the Bank of Mexico said Wednesday. The amount of funds involved in the irregular activity totaled "approximately 300 million pesos ($15.3 million)," Central Bank Governor Alejandro Diaz de Leon told reporters, according to Agence France Presse. He said commercial bank customers accounts were never in danger. An investigation is underway, the governor said, without indicating if the suspected hackers were domestic or international. He declined to identify the companies that had been hit, only saying that three banks, a broker and a credit union had seen fake transfers. Sources close to the investigation told Reuters that there were cash withdrawals from dozens of banks around the country shortly after hundreds of fraudulent transfers. The interbank payments system allows banks to make real-time transfers to each other. They connect via their own computer systems or an external provider -- the point where the attacks appear to have taken place, Lorenza Martinez, director general of the corporate payments and services system at the central bank, said on Monday. Martinez revealed that at least five attacks had occurred but, at that time, said the amount taken was still being analyzed. After the attacks were detected, banks switched to a slower but more secure method, AFP said.