One of the mysteries surrounding the disappearance of a Malaysia Airlines jetliner on a flight from Kuala Lumpur to Beijing is the appearance of two men on the flight manifest who were apparently traveling with stolen passports.

On U.S.-bound flights there are safeguards aimed at preventing that from happening. Interpol, the international police organization, issued a statement criticizing Malaysia for allowing the passengers to board the flight.

Since 9/11, Interpol has maintained a database of stolen passports containing more than 40 million entries, according to the agency. Airlines and nations are supposed to check passenger lists against that database to prevent people from flying with false identification. But relatively few nations do.

Among those that do is the United States. Former Assistant Homeland Security Secretary Stewart Baker helped institute the policy.

"We said to the carriers, the airliners, 'Before you close the door and take off on your flight to the United States, we already have to have all the airline passengers' names and passport numbers in our hands electronically,' so that we can make a decision to say, 'Don't take off. There's somebody on that plane we don't want coming here," Baker says.

Baker says countries who's citizens don't need visas to enter the U.S. must report stolen passports to Interpol within 24 hours, and the passport numbers of passengers flying here must be sent online to the U.S. in advance of the flight departing.

"We've got redundant checks that ought to make it very difficult to take off on a flight to the U.S. with a properly reported stolen passport, as these evidently were," he says.

The U.S. and Britain are among the few nations that do regularly check for stolen documents. Interpol says passengers were able to board aircraft more than a billion times last year alone without having their passports checked against its databases.

Michael Greenberger, director of the University of Maryland's Center for Health and Homeland Security, says the system is broken.

"The world passport system is in total disarray, and the conventional wisdom that somehow a passport is going to keep unwanted people out of a country is just a fiction," Greenberger says. "It is far too easy to get into a country with illegal documents."

While the presence of two people with stolen passports on the Malaysia Airlines flight has drawn attention to the issue, Greenberger says the use of stolen or forged passports is rampant and has serious implications:

"This use of fake passports vastly increases organized crime, drug trafficking, human trafficking," he says.

A 2011 Government Accountability Office report says the use of fake passports represented a key gap in the abilities of different countries to prevent terrorist travel overseas.

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