United States to step up international student visa scrutiny WASHINGTON, May 3 (UPI) -- The Department of Homeland Security has ordered border agents to verify the validity of visas for all foreign students entering the United States, CNN said. Citing a U.S. government official it did not identify, CNN reported Friday the directive was contained in a memo distributed this week. The official said the order, effectively immediately, is part of an effort to make sure national security officials are up-to-date in real time on student visa information. There were reports this week Dias Kadyrbayev and Azamat Tazhayakov -- charged with offenses, including obstruction of justice, in the Boston Marathon bombing attack -- were in the United States on expired student visas. Officials haven't linked either of the two men -- foreign exchange students from Kazakhstan at the University of Massachusetts-Dartmouth, where their friend, bombing suspect Dzhokhar Tsarnaev, was enrolled -- to the Boston attack but say they hope they can learn more about the suspects' movements before and after the marathon, a source told CNN. "These guys are not being cut loose immediately, and there's a reason why," the federal law enforcement source said.