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At the helm of Citigroup for just a year and a half, Michael L. Corbat has been trying to transform into a boring bank a global giant that has been plagued in the past by blowups and bailouts.

Now a scandal at the bank’s Mexican subsidiary shows that the chief executive still has work to do, as the development revives some familiar concerns about the sprawling bank’s ability to manage risk.

Citigroup said on Friday that it had recently uncovered fraud in its Mexican banking unit, Banamex, forcing the bank to restate its 2013 earnings.

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Citigroup said as much as $400 million was misappropriated in the fraud. In a harshly worded memo to employees, Mr. Corbat said it was unclear how many people were involved in the activity, which centered on the oil services company Oceanografía.

People briefed on the matter said that at least one Banamex employee was suspected of enabling the fraud. Citigroup said it was working with investigators in Mexico to “initiate criminal actions” that might yield “just penalties on the responsible parties” and could allow the bank to recover damages.

The Mexican government seized control of Oceanografía’s assets Friday morning, a move that the attorney general, Jesús Murillo Karam, said at news conference in Mexico City was meant “to preserve jobs” and “the company’s documents.”

A big question is why Citigroup was doing business with Oceanografía in the first place.

The company is well known among Mexican investors as politically connected but financially shaky. It supplies marine engineering services and derives nearly all of its business from Pemex, Mexico’s government-owned oil monopoly.

“This company has been toxic for a long time,” said Luis Maizel, senior managing director of LM Capital Group, which invests in emerging market debt.

The United States ratings firm Fitch warned about Oceanografía’s high leverage and poor cash flow generation in 2009. The next year, Fitch eventually withdrew its ratings because the company was not supplying enough information. In 2008, Standard & Poor’s noted that Mexico’s congress had investigated allegations of improper deals between Oceanografía and Pemex, though no wrongdoing was proved.

The latest scandal comes at an awkward time, as Mexico is getting ready to open its closed energy industry to outside private investors.

Citigroup, through Banamex, provided credit to Oceanografía in several ways. It extended $585 million of short-term credit through an accounts receivable financing program.

The program typically worked like this: Banamex would advance money to Oceanografía to provide services to Pemex. The oil giant would then pay back Banamex, verifying invoices provided by Oceanografía to confirm that the work was completed.

In theory, Banamex was relying on Pemex’s rock-solid ability to pay back the bank, which made the transaction the equivalent of a government-guaranteed loan.

But Banamex also lent $33 million directly to Oceanografía in the form of loans and standby letters of credit.

“When you have a company getting contracts from the government, it looks like a very attractive credit to a bank,” Mr. Maizel said. “But in Latin America, you are lending to the people running the companies, not just the companies.”

Mr. Corbat’s reputation rests, in part, on his ability to lead Citigroup into an era when it is free from damaging incidents. But Citigroup’s far-flung operations could complicate his efforts to keep a tight lid on employee misconduct. And the Mexican market is an important one for the bank, accounting for about 13 percent of its revenue, according to a Credit Suisse analysis.

In the Mexico case, analysts and even some regulators privately praised Mr. Corbat’s public response. He vowed to hold accountable the people behind the fraud.

“I can assure you there will be accountability for those who perpetrated this despicable crime and any employee who enabled it,” Mr. Corbat said in a memo to employees. “All will be held equally responsible, and we will make sure that the punishment sends a crystal-clear message about the consequences of such actions.”

Federal authorities in the United States are also scrutinizing what happened in Mexico. According to one person briefed on the matter, the bank has provided briefings for federal regulators in New York and Washington, who are examining whether lax controls allowed the scheme to unfold.

The Securities and Exchange Commission and the F.B.I. in New York are also preliminarily reviewing the conduct, another person said.

Citigroup has been in Mexico since 1929, and it has been a favorite institution of the Mexican elite. When the country nationalized the banks in 1982, Citigroup was allowed to remain in Mexico even while most foreign banks had to leave. In 2001, it acquired Banamex in a $12.5 billion deal.

The situation in Mexico is an echo of past problems in Latin America that saddled Citigroup with huge losses. In the 1980s, the bank was crippled by bad loans in the region.

The recent scheme began to unravel, according to Citigroup and Mexican officials, when Pemex found irregularities in the bonds that Oceanografía was required to put up to guarantee the completion of its contracts.

In response, Mexico’s federal comptroller suspended Oceanografía on Feb. 11 from entering into new government contracts for 20 months.

That move led Banamex to review its $585 million financing program to Oceanografía. During that review, the bank determined that a significant portion of the accounts receivable were fraudulent.

The bank said it appeared that invoices from Oceanografía were falsified to represent that Pemex had approved them. The Banamex employee who processed them is suspected of being involved in the fraud, people briefed on the matter said.

Citigroup is not the only major lender doing business with Oceanografía. The company has borrowed hundreds of millions of dollars from the capital markets in recent years.

A $335 million bond issue in 2008 helped finance the acquisition of new vessels, and the company now owns the largest offshore construction fleet in Mexico, with 69 ships.

The company does not have a track record of treating bondholders very well, said Mariela Anguiano, an analyst who follows the company for BCP Securities in Greenwich, Conn. They tended to pay coupon payments on the last possible day, she said, and did not provide much information.

In a prospectus issued late last year for the sale of $160 million in bonds, the company stated: “The group is from time to time subject to various accusations, including accusations of corrupt practices.”

“When the underwriter came by my office trying to sell me on the bonds, I said, ‘How dare you offer this to me?’ ” said Carlos Legaspy, president and chief executive of InSight Securities, which invests in corporate and distressed debt in Latin America.

For years, the rapid growth of Oceanografía and other well-connected local contractors has raised concerns about the cozy relationship between the government oil monopoly and its long-term suppliers.

Pemex’s chief executive, Emilio Lozoya Austin, a close associate of President Enrique Peña Nieto’s, has promised a widespread sweep of corruption in the company, ahead of forcing it to compete or ally with private investors.

Pemex said on Friday that Oceanografía’s alleged irregularities were “an isolated case” and that it was continuing business as usual.

Ben Protess and Peter Eavis contributed reporting.