More than a decade after the 9/11 attacks, air travelers have learned to contend with measures including full-body scanners and limits on carry-on liquids. But travel documents may still be a weak link.
Investigators are working to determine if stolen passports played a role in the disappearance of Malaysia Airlines Flight MH370, which vanished Saturday en route from Kuala Lumpur to Beijing with 239 people aboard. Interpol confirmed Sunday that at least two passports reported as stolen, one from Austria and one from Italy, were used to board the flight.
"While it is too soon to speculate about any connection between these stolen passports and the missing plane, it is clearly of great concern that any passenger was able to board an international flight using a stolen passport listed in Interpol's databases," Ronald K. Nobles, the secretary general, said in a statement.
Connection or no, experts say fraudulent passports can represent a significant security risk, particularly abroad.
"There needs to be a better job of watching for the use of lost and stolen passports," said Stewart Baker, assistant secretary for policy at the Department of Homeland Security from 2005 to 2009. There are straightforward procedures in place to check passports against Interpol databases and other records before passengers board a flight. "We in the U.S. check it frequently, at various stages," said Baker, a partner at Washington-based law firm Steptoe & Johnson. But many other countries simply don't.
According to Interpol, few of its 190 member countries systematically search agency databases to see if a passenger is using a travel document reported lost or stolen. By its estimates, those lapses mean passengers were able to board planes more than a billion times last year without having their passports screened.
The two passports used to board Flight 370 were reported stolen in Thailand, in 2012 and 2013, according to Interpol. Neither had been checked against the database in that time. NBCNews reports that the tickets for those passengers were purchased with cash by an Iranian middleman,and that the FBI is expected to compare the passengers' fingerprints against its database.
(Read more: Thailand struggles with booming fake passport market)
United States has put more precautions and stricter standards in place since 9/11. "This couldn't happen on a flight to the United States," said Baker.
An international flight bound for the United States requires airlines to confirm a visa, visa waiver authorization or U.S. passport for each passenger, said C. Stewart Verdery Jr., who served as assistant secretary for policy and planning at DHS from 2003 to 2005.
Travelers applying for a visa or waiver would have their documents vetted at application, including a check against the Interpol database of stolen documents, and checked again at the airport to ensure the passport used to apply is the one presented to board, he said.
Airlines are also required to check travel documents against the so-called no fly list of high-risk passengers.
Both passengers who boarded Flight 370 with stolen passports were booked to continue on flights to Amsterdam, The Wall Street Journal reported. Verdery said the U.S. requires similar identity checks regardless of whether the country is the travelers' end destination or merely a stopover.
"All this has been reworked substantially since 9/11," he said. "We've built a lot of new security protocol to try to reduce the risk."
Advocates including the 9/11 Commission have recommended the adoption of biometric passports, which would be difficult to fake or alter. Consumers are also likely to see a push for more participation in travel screening programs such as TSA PreCheck or the U.S. Customs and Border Patrol's Global Entry program, said John Hernandez, a senior industry analyst with Frost & Sullivan. Such programs are now commonly used to shorten screening wait times, and could become more crucial as a background check.
(Read more: Five things to know about the missing jet)
But security expert Bruce Schneier said if stolen passports did play a role in the Malaysia Airlines disappearance, it doesn't necessarily point to a wider security risk. "How many millions of people fly every day?" he said.
Instead of implementing more measures in response to a specific kind of plot or threat, airport security should be more focused on "intelligence, investigation and emergency response," he said. Such broad measures would likely offer travelers better protections, without adding to airport hassles. "It would all be behind the scenes," Schneier said. "It would just be people doing their jobs."