Three people were arrested Wednesday on charges of running four sham vocational schools in Southern California that issued fraudulent paperwork enabling foreigners to stay in the country and netted as much as $6 million a year.

Hee Sun Shim, 51, of Beverly Hills; and Hyung Chan Moon, 39, and Eun Young Choi, 35, of Los Angeles, were arrested on a 21-count indictment, the U.S. attorney's office said in a statement.

Authorities said students from South Korea and China who were previously enrolled elsewhere transferred to Prodee University and related schools but never attended classes, paying up to $1,800 for a six-month enrollment to get fraudulent paperwork in support of their student visas.

Federal investigators said they were tipped off after inspectors made an unannounced visit to Prodee in 2011 and saw only one class in session with three students, though records showed hundreds were enrolled.

"We had students who were hundreds, thousands of miles away," said Claude Arnold, special agent in charge of homeland security investigations for Immigration and Customs Enforcement in Los Angeles. "There was absolutely no education going on at the school."

It was not immediately clear whether the defendants had attorneys who could respond to the allegations. They were due to make their first court appearance Thursday.

No one answered at two phone numbers listed for Prodee University.

Federal authorities have been cracking down on schools they say solely operate to help foreigners procure student visas. Last year alone, immigration authorities flagged 150 of some 9,000 schools certified to accept foreign students for investigation, many of them in California.

Government watchdogs have said the recent fraud cases exposed gaps in immigration authorities' oversight of schools that admit foreign students

In this case, authorities are in the process of stripping Prodee and affiliated schools' ability to enroll foreign students. Students will be allowed to return to their countries or present legitimate documents to support their visas, Arnold said, adding that some would remain as material witnesses in the case.

The school created fake Los Angeles-area addresses for foreigners to use on their paperwork and fabricated attendance records and transcripts on their behalf, according to a copy of the indictment. In exchange, the defendants charged "tuition" and told foreigners to lie to immigration officials and say they attended classes, court papers said.

Authorities said Moon and Choi worked for Shim. All three face conspiracy and fraud charges. Shim is also charged with encouraging illegal residence and money laundering, the statement said.

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Associated Press writer Sudhin Thanawala in San Francisco contributed to this report.