Updated

Civil liberties advocates say they've discovered a government program to screen immigrants for national security concerns that has blacklisted some Muslims and put their U.S. citizenship applications on hold for years.

The American Civil Liberties Union of Southern California says immigration officers are instructed to find ways to deny applications deemed to be a national security concern, for example, by flagging discrepancies in a petition.

The ACLU said Wednesday that the criteria used to blacklist immigrants are overly broad and include traveling through regions where there is terrorist activity.

It isn't clear how many immigrants have been reviewed under the program.

U.S. Citizenship and Immigration Services spokesman Christopher Bentley says the agency routinely checks the background of immigrants applying for benefits and will not sacrifice public safety to expedite applications.