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.