The US has expressed concern over any activity involving visa fraud, as several Indian students of a California-based University shut down by authorities on charges of illegal immigration were interrogated.Duped by the authorities of Tri-Valley University, which has now been shut down, hundreds of Indian students, mostly from Andhra Pradesh, faced the threat of being deported back home after having lost their student visa status.According to a federal complaint filed in a California court last week, the University helped foreign nationals illegally acquire immigration status.The university is said to have 1,555 students. As many as 95 per cent of these students are Indian nationals. The students are reported to have paid lakhs of rupees for obtaining a visa for their category and also for students work permit.Meanwhile, reports from the San Francisco area said a number of students have been interrogated by federal authorities as part of their investigation against the Tri-Valley University, which is being charged of part of an effort to defraud, misuse visa permits, and indulge in money laundering and other crimes.Investigations by Immigration and Custom Enforcement (ICE) found that while students were admitted to various residential and on-line courses of the university and on paper lived in California, but in reality they illegally worked in various parts of the country as far as Maryland, Virginia, Pennsylvania, and Texas.