MEXICO CITY (AP) — Mexico's banking system has somehow misplaced between $18 million and $20 million in electronic transfers between banks, authorities said Monday, the latest in a series of embarrassing breakdowns that have affected debit card purchases and e-payments across the country.

Mexico's central bank and regulatory agencies said they are not sure whether the problem with settlement transactions among banks was the work of outside hackers, an inside scam or errors.

But in a country where phishing emails and freelance debt collectors often use banks' logos and letterheads, it is no secret that bank security standards are lax.

Depositors won't be affected, but the banks themselves could take a hit on the missing money.

"This is a tough lesson that these standards must be tightened," said Mario Di Costanzo, who heads the government commission to protect financial customers.

Di Constanzo said that "for some time, we have noticed that banks in general have to do more, not just to protect the information of their customers, but to protect their own identities."

It wasn't exactly clear how the shadow transaction occurred. Di Constanzo said only that "the identity and recipients of some transactions made through the SPEI (Mexico's official electronic payment system) have not been identified."

He said the amount not accounted for may be between 350 million pesos and 400 million pesos.

The shortfall was discovered only after banks went into security mode following the detection of software vulnerabilities at three banks on April 27.

After the vulnerability was detected, the three banks abandoned the outside contractor that supplied the defective software and changed over to the central bank's own settlement system.

The central bank then required all banks to take additional security measures, slowing some time-sensitive transactions — like debit card purchase approvals and e-payments — to a crawl. In a society that is starting to transit toward a more cashless economy, that meant agony for some retail customers and ATM users.

Apparently, problems continue.

Citibank's Mexico subsidiary, Citibanamex, acknowledged some problems with debit cards that worked only "intermittently" over the weekend.

"We regret any inconvenience caused to our customers' operations," the bank said in a statement.