to my husband, is it because we're muslim and my husband said why would you say that? and she goes, well, because we were the only muslim pop in that line. >> reporter: u.s. customs and boarder protection said quote, the religion faith or spiritual beliefs of an international traveller are not determining factors. for flights in or out of the united states, passenger security checks happen long before travelers get to the airport, even before the airline accepts payment, names are run against the government's no-fly list. if there is a hit, the sale is denied. for those who've bought a ticket, names are continuously checked against other government databases searching everything from past itineraries to law enforcement and intelligence information. if there is a problem, the airlines deny boarding at the airport. but it's not just terrorism. there are other reasons a passenger could be denied boarding like incomplete documents or health-related issues. >> that was cnn's rene marsh reporting.