The Homeland Security Department has investigated whether Iranian operatives exploited a controversial U.S. visa program for immigrant investors, according to a DHS memo revealed on Thursday. The EB-5 program has existed since 1990 but has exploded in popularity in recent years. It allows foreigners to obtain residency status, or a green card, for a $1 million investment in the United States. The FBI and other agencies have raised security concerns about the program. Some critics say the Chinese government and other foreign agents could use the program for espionage and other purposes.