The former owners of a California flight school have pleaded guilty in a massive visa fraud scheme in which they hired illegal immigrants as flight instructors — a scam that immigration officials say posed a "significant" threat to national security.

Andrew Burr and Christopher Watson, the former president and vice president of Anglo-American Aviation, pleaded guilty Thursday to misdemeanor counts of hiring unauthorized workers, part of a larger felony case against their company that was the culmination of a two-year investigation by Immigration and Customs Enforcement.

ICE officials found that from 2001-2008, the company hired 11 illegal immigrants as flight instructors.

"The actions by the defendants in this case not only undermined the integrity of our nation's legal immigration system, they also posed a significant national security risk," said Joe Garcia, acting special agent in charge for the ICE Office of Investigations in San Diego.

ICE investigators focused on a four-month period in the summer of 2007, when the school issued visa documents to more than 100 foreign students, even though the Federal Aviation Administration had revoked their certification to train commercial pilots.

The company was abusing an online database developed in the wake of 9/11 to track foreign students, according to ICE officials. Without that FAA certification, the company was forbidden to issue I-20 immigration documents to its students, who used the forms to get visas from the State Department.

ICE investigators successfully tracked down 53 of the students — more than half of those trained in the summer of 2007, according to ICE statements — and said none were found to pose a security risk. They provided no information on those they failed to locate.

At the hearing Thursday, Burr and Watson were sentenced to five years' probation, and along with the flight school must forfeit $250,000 in illegal profits.

Anglo-American Aviation Inc. also pleaded guilty to felony charges for making false statements and creating false visa documents, and will be sentenced on June 21.

The company now operates under a different name — American Aviation Academy — but uses the same mailing address and phone number in San Diego, where two of the 9/11 hijackers received flight training