It was not clear how much of the money transferred was later withdrawn in cash. Some of the attempts to fraudulently transfer funds were blocked, the sources said.

Mexico’s central bank Governor Alejandro Diaz de Leon told journalists late Monday that the attack on the payment system was unprecedented and that he hoped that measures being taken would stop future incidents.

“There’s no evidence that would allow us to say with certainty that this is over,” he said. “We’re taking corrective and mitigating action.”

He later said in a radio interview that all the evidence, which is so far only partial, pointed to a cyberattack. Lorenza Martinez, head of Banxico’s payment system, told Reuters on Friday that five institutions saw “unauthorized transfers.”

Inter-bank transfers slowed in later April, feeding worries that Latin America’s second biggest economy could be the latest victim in a global wave of cyber attacks.

“In terms of the security of the bank’s offices, I think that is part of the analysis that each bank is doing,” Martinez said.

He said that the central bank’s SPEI interbank transfer system was not compromised but that the problem had to do with software developed by institutions or third-party providers to connect to the payment system.

Many banks have migrated to an alternate, slower technology to connect to the payment system, she said.

The central bank also said that no clients had been affected so far. Martinez said that the transfers hit accounts of financial institutions in the central bank.

Source: Reuters news agency