How did stolen passports get through?

A member of the Vietnamese air force checks a map during continued search operations for a Malaysia Airlines jet near Phu Quoc Island off the coast of Vietnam on Tuesday, March 11. Contact with the Boeing 777-200 was lost as it flew over the South China Sea early Saturday after leaving Kuala Lumpur for Beijing. It was carrying 227 passengers and 12 crew members.

Iranians Pouria Nour Mohammadi, second left, and Delavar Seyed Mohammad Reza, right, were identified by Interpol as the two men who used stolen passports to board the flight. Officials said it is unlikely that Mohammadi, 18, and Reza, 29, seen here in an image submitted by an iReporter, were part of a terrorist group.

An Indonesian navy crew member scans an area of the South China Sea bordering Indonesia, Malaysia and Thailand on Monday, March 10.

Vietnam air force Col. Le Huu Hanh is reflected on the navigation control panel of a plane that is part of the search operation over the South China Sea on March 10.

Relatives of the missing flight's passengers wait in a Beijing hotel room on March 10.

A U.S. Navy Seahawk helicopter lands aboard the USS Pinckney to change crews before returning to search for the missing plane Sunday, March 9, in the Gulf of Thailand.

Members of the Fo Guang Shan rescue team offer a special prayer March 9 at Kuala Lumpur International Airport in Sepang, Malaysia.

A handout picture provided by the Malaysian Maritime Enforcement Agency shows personnel checking a radar screen during search-and-rescue operations March 9.

Italian tourist Luigi Maraldi, who reported his passport stolen in August, shows his current passport during a news conference at a police station in Phuket island, Thailand, on March 9. Two passengers on the missing Malaysia Airlines flight were reportedly traveling on stolen passports belonging to Maraldi and an Austrian citizen whose papers were stolen two years ago.

Hugh Dunleavy, commercial director of Malaysia Airlines, speaks to journalists March 9 at a Beijing hotel where relatives and friends of the missing flight's passengers are staying.

Vietnamese air force crew stand in front of a plane at Tan Son Nhat airport in Ho Chi Minh City, Vietnam, on March 9 before heading out to the area between Vietnam and Malaysia where the airliner vanished early Saturday.

Buddhist monks at Kuala Lumpur International Airport offer a special prayer for the missing passengers on March 9.

The Chinese navy warship Jinggangshan prepares to leave Zhanjiang Port early on March 9 to assist in search-and-rescue operations for the missing Malaysia Airlines flight. The Jinggangshan, an amphibious landing ship, is loaded with lifesaving equipment, underwater detection devices and supplies of oil, water and food.

Members of a Chinese emergency response team board a rescue vessel at the port of Sanya in China's Hainan province on March 9. The vessel is carrying 12 divers and will rendezvous with another rescue vessel on its way to the area where contact was lost with Malaysia Airlines Flight 370.

The rescue vessel sets out from Sanya in the South China Sea.

A family member of missing passengers is mobbed by journalists at Kuala Lumpur International Airport on Saturday, March 8.

A Vietnamese air force plane found traces of oil that authorities had suspected to be from the missing Malaysia Airlines plane, the Vietnamese government online newspaper reported March 8. However, a sample from the slick showed it was bunker oil, typically used to power large cargo ships, Malaysia's state news agency, Bernama, reported on March 10.

Malaysian Prime Minister Najib Razak, center, arrives to meet family members of missing passengers at the reception center at Kuala Lumpur International Airport on March 8.

Malaysia Airlines official Joshua Law Kok Hwa, center, speaks to reporters in Beijing on March 8.

A relative of two missing passengers reacts at their home in Kuala Lumpur on March 8.

Wang Yue, director of marketing of Malaysia Airlines in China, reads a company statement during a news conference at the Metro Park Lido Hotel in Beijing on March 8.

Chinese police at the Beijing airport stand beside the arrival board showing delayed Flight 370 in red on March 8.

A woman asks a staff member at the Beijing airport for more information on the missing flight.

A Malaysian man who says he has relatives on board the missing plane talks to journalists at the Beijing airport on March 8.

Passengers walk past a Malaysia Airlines sign on March 8 at Kuala Lumpur International Airport.

Malaysia Airlines Group CEO Ahmad Juahari Yahya, front, speaks during a news conference on March 8 at a hotel in Sepang. "We deeply regret that we have lost all contacts" with the jet, he said.

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• Brian Havel: It's not surprising that at least two on Malaysia flight had stolen passports


• He says how passengers treated on security lines varies with nations' regulations


• He says many Interpol member nations don't use database -- even U.S., EU differ on privacy


• Havel: It's time for global coordination, standardization for processing travel documents

Editor's note: Brian Havel is distinguished research professor of law, associate dean for international affairs and director of the International Aviation Law Institute at DePaul University College of Law. He is visiting lecturer in law at the University of Oxford, Wadham College. He serves on the World Economic Forum's Aviation, Travel and Tourism Council and the board of the European Air Law Association.

(CNN) -- The Malaysia Airlines mystery remains unresolved. While it is too early to speculate about the cause of the airliner's disappearance, it is not a surprise that at least two people were able to board Malaysia Flight 370 with stolen passports.

The fact is the world is far from having a foolproof system for checking travelers who move across borders.

More than 50% of transborder passenger movements take place at airports, but how passengers are treated in security lines at the world's 600-plus major international airports is very much dependent on local regulations and practices. Various international organizations have proposed so-called "smart" solutions, such as biometric passports and electronic visas, but many forms of documentation used every day by millions of airline passengers are not digitized and are not being checked against any international databases.

Moreover, without biometric protection, it is always possible to tamper with conventional passports, including the passport holder's photograph, something that might have happened here. Of course, there are some important exceptions where tighter controls prevail.

In the European Union, the nations that belong to the "Schengen" agreement, which abolished internal borders among 27 European countries, use a shared security database for travelers moving within the Schengen-area member states. The United States, the United Kingdom and the United Arab Emirates are the biggest users of the Interpol database for lost or stolen travel documents.

Importantly, too, the United States requires airlines entering and leaving U.S. territory to submit travel document information about all passengers. Nevertheless, it remains true, even in a world that values security, that nations continue to have their own ideas about how to monitor travelers, how security information should be shared and to what extent identity information should be the subject of strong privacy protection.

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What happened to Malaysia Flight 370?

Search for Flight 370 expands

Most of the 190 nations that belong to Interpol, for example, simply do not bother to use the database even though they have access to it. As to privacy, even the United States and EU have different approaches, with the EU treating personal identity information as part of human and civil rights, while the U.S. authorities seem to have taken the view that individual privacy is almost always subject to national security concerns.

Diversity among nations' practices, therefore, can produce situations where travel document problems are missed or overlooked. Malaysia is far from being the only country where the passports of outgoing passengers are not typically checked. While we do not yet know the exact sequence of events, it is possible that China would have checked the stolen passports had it been required to issue visas. But no visas were required because the tickets were for continuing travel to Amsterdam, Netherlands.

It is possible, though not of course yet certain, that existing procedures meant that Malaysia in effect passed responsibility to China and that China deferred to the Netherlands. Thus, all three nations probably complied with their own procedures and were not in violation of any international law or practice.

The lack of alignment among these various nations' procedures, however, meant that the passengers could use stolen travel documents to proceed through several security lines.

The lessons from all of these national variations are clear: First, nations that develop their own security databases should still cross-check them with existing international systems such as Interpol. And second, the time has come for global coordination and standardized procedures for processing travel documentation: that includes not only widespread adoption of biometric travel documents, but also better coordination among nations as to where responsibility lies for checking documents that travelers use to identify themselves.

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The opinions expressed in this commentary are solely those of Brian Havel.