FORTUNE — Citigroup C has fired 11 additional workers – four of which are senior executives – over alleged accounting fraud at its Mexican unit, Banamex.

Among the four senior executives, two led businesses in Mexico. A twelfth employee had been fired previously because of the fraud claims.

“We have unfortunately identified additional employees across business and functional lines whose actions or inactions failed to protect our company from this fraud,” wrote Citigroup Chief Executive Officer Michael Corbat in a memo to employees.

Corbat, who went to Mexico this week, added that there may be further repercussions. “Before our investigation concludes, we expect that several other employees, both inside and outside Mexico, may receive forms of disciplinary action as well,” he added.

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The fraud relates to a loan that Banamex extended to the oil services company Oceanografia – a loan that the company could not pay back.

Oceanografia had reportedly falsified invoices to Pemex, the Mexican oil giant, that were used as collateral for the loans.

In February, the bank said that it had lost about $400 million. A Reuters story notes that Citigroup has since found additional fraudulent loans linked to Oceanografia and another unidentified oil services company, bringing total losses to $565 million.