Federal investigators are cracking down on what they say are sham universities that exploit the visa system for foreign students.

At least half a dozen schools have been shut down or raided by federal authorities in recent years over allegations of immigration fraud. Like Tri-Valley University in Pleasanton, California, they had obtained permission from immigration officials to admit foreign students.

But investigators say most offered little or no instruction or didn't require all students to attend classes, instead taking advantage of the visa system for profit.

Government watchdogs say the recent visa fraud cases have exposed gaps in U.S. Immigration and Customs Enforcement oversight of schools that admit foreign students — a problem the agency says is being corrected.

Experts say they also hurt the U.S. higher education system's reputation.