Wendy Koch and Rick Hampson

USA TODAY

International travelers be advised: almost 13 years after the 9/11 attacks, in a world awash in the kind of bogus passports that helped that plot succeed, your travel document remains easy to steal, replicate and adulterate -- and often no one's checking.

"Passports are a very weak link'' in the world's travel security system, Michael Greenberger, a former Clinton administration official and director of the University of Maryland Center for Health and Homeland Security, said Sunday.

His comments came as investigators tried to determine if stolen passports used by two passengers played a role in Saturday's disappearance of a Malaysia Airlines flight bound for China with 239 on board.

This much was clear: Although there are millions of passports listed as stolen, lost or missing in the world, only a few countries systematically check for them at airport gates.

And Greenberger said that despite concerns raised by the 9/11 Commission in its report in 2004, there's still no effective way to ensure that the person presenting a passport is the one to whom it was issued.

More than 40 million travel documents, mostly passports, have been reported stolen or missing, according to a database begun in 2002 by the international law enforcement organization Interpol.

Yet, "only a handful of countries worldwide are taking care to make sure that persons possessing stolen passports are not boarding international flights,'' Interpol Secretary General Ronald Noble said Sunday.

Interpol said that no country checked the two passports used to board the Boeing 777 bound from Kuala Lumpur to Beijing, even though they were reported stolen in Thailand -- an Austrian one in 2012 and an Italian one in 2013—and that it can't say how many other times they might have been used.

The U.S. uses Interpol's database more than any other nation to screen people entering the country. Its 250 million annual checks are followed by the United Kingdom's 120 million and the United Arab Emirates' 50 million.

Interpol said it makes its database available to all 190 member countries but cannot force them to integrate it into their own systems. Last year, passengers boarded planes more than a billion times without having their passports screened against its database.

Each year, based on its terrorist watchlist and Interpol data, the U.S. government issues thousands of "no-board recommendations" to airlines to keep suspected high-risk passengers from traveling to the United States. In fiscal year 2011 alone, it issued more than 3,600 such recommendations, according to 2012 U.S. House testimony by Kevin McAleenan of the Department of Homeland Security's Customs and Border Protection.

The black market for fake passports is geared mostly to smuggling laborers, prostitutes and other illegal migrants. But such documents also have been used by terrorists, including Ramzi Yousef, convicted of carrying out the 1993 World Trade Center bombing.

The 9/11 Commission report detailed how the hijackers obtained and modified the passports that got them into the U.S. It also listed five ways terrorists can use passports, including changing stolen ones with new photos or doctoring them to create a fake travel history by adding or removing visa entry stamps.

The report advocated advanced biometric passports – difficult to replicate and intrinsically linked to whomever it was issued. But, because of budget concerns, "the federal government keeps the system we have in place and hopes nothing bad happens," Greenberger said.

Still, he said, the U.S. system, which involves checking reported stolen or false passports against its own watchlist, is better than the one used internationally. Had the Malaysia Airlines flight been headed to the United States, he said, the passenger manifest would have been checked before takeoff, primarily against the U.S. watchlist.

Since many nations neither maintain their own watchlists nor check any list as carefully as the U.S, "if you're flying between two foreign airports, you're at the mercy of whatever the host and receiving countries are doing,'' Greenberger said.

Glenn Winn, an aviation security consultant and former head of security at United and Northwest airlines, said that a fake passport's quality is almost beside the point if authorities "don't even bother to check it or swipe it.''