Nov. 6, 2013  -- An official with the U.S. Embassy in Vietnam has admitted he rubber-stamped nearly 500 visitor visas to the United States in exchange for more than $3 million in bribes, which he then used to buy nine properties overseas.

Michael T. Sestak, 42, pleaded guilty in Washington Wednesday, telling a federal judge he was part of a conspiracy – along with five others from the U.S. and Vietnam – to commit bribery and visa fraud.

From 2010 to 2012, Sestak was stationed in Ho Chi Minh City, where he was in charge of reviewing and approving visa applications. It was "a sensitive position," as the Justice Department called it.

Under the conspiracy, a group of "agents" would recruit customers and advertise a "deal" online, promising Vietnamese nationals that their non-immigrant visa applications to the United States would be approved, even if it was an application that had previously been denied, according to the Department of Justice. The cost: As much as $70,000 per application.

The group allegedly helped applicants fill out their applications and tutored them over the required in-person interviews, offering sample questions and suggested answers. The interviews, however, were conducted by Sestak, who then approved visas for paying customers.

"This Foreign Service Officer corrupted the integrity of a process designed to screen visitors to the United States, a process that obviously has implications for our national security," U.S. Attorney Ron Machen said in a statement. "His motivation for betraying his oath of office was cold, hard cash."

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Evidence uncovered by authorities suggests Sestak was essentially recruited himself, with his co-conspirators targeting him early on and "purposefully cultivat[ing]" a relationship with him "so then later he can do [one of us] favors like … approve visas for people," as one co-conspirator put it in an online chat cited by federal prosecutors.

"He's the guy that approves Visas for Vietnamese people to go to the United States so he's a really good connection to have," the co-conspirator said of Sestak in an online chat dated June 1, 2012. "My brother plans on using him to get [a relative's visa approved]."

The entire scheme generated at least $9,780,000. Sestak personally received about a third of that, which he laundered through China into Thailand, where he bought nine real estate properties, according to federal prosecutors.

Ironically, in the midst of the scheme, the State Department was promoting an event in Ho Chi Minh City hosted by Sestak.

"Secrets of the visa application process revealed!!... Which day is the luckiest one to apply for a visa? What are the secret best answers to the visa interview questions? Where can I get the documents that will guarantee that I receive a visa?" said a State Department flyer advertising the March 22, 2012, event.

U.S. authorities first learned of the bribery scheme after a confidential source sent a letter to the U.S. consulate in Ho Chi Minh City. The letter outlined the scheme and included the names, birth dates and even photographs of seven people who obtained visas through it.

The State Department's Diplomatic Security Service immediately launched an investigation.

Sestak and one of his alleged co-conspirators, a U.S. citizen living in Vietnam, were arrested in May. Two others have also been taken into custody. But one of those allegedly involved in the bribery scheme – the wife of another alleged co-conspirator -- remains at large in Vietnam. A warrant has been issued for her arrest.

"This case demonstrates how cooperation with State Department partners in the region allowed the Department of Justice to pursue charges where Vietnamese citizens were victimized by individuals guided by greed," said U.S. Department of State Diplomatic Security Service Director Gregory Starr.

No date has been set for Sestak's sentencing. He faces more than 24 years in prison.

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