Your lost or stolen passport may have found a new life in the shady underworld of a crime gang or in the pocket of a terrorist plotting an attack. Like crime syndicates, terrorist networks are often globe-spanning operations, and doctored documents are the keys to unlocking borders and staying below the radar.

Fighting bogus identities is now a top security priority, with France's interior minister pushing for Europe-wide action, and Interpol pleading for tougher document policing as it warns that the world is awash in 38 million lost or stolen passports that are ripe for doctoring.

False identities have complicated the task of investigators trying to untangle the many-tentacled and overlapping networks that carried out the recent attacks in Paris and Brussels. Many of the attackers used fake identities.

Investigators spotted the trail of Najim Laachraoui after he built the bombs used in the Paris bloodbath — but couldn't stop him before he blew himself up at the Brussels airport four months later because they knew him only from his fake Belgian ID under the name of Soufiane Kayal.

A fraudster's den discovered in a Brussels suburb indicates the scale of the crime, and how hard it is to vanquish: It held some 1,000 digital images used to make false documents. Weeks after authorities raided the site in October, three people connected to those documents joined in the attacks on Paris. It was four months before the chief fraudster was arrested — across several European borders, in Italy.

"The central element for the secrecy of organizations is to have false passports and false identity papers. It is absolutely indispensable," said retired anti-terrorism judge Jean-Louis Bruguiere, once France's top counter-terrorism investigator.

The underworld of document doctors provides many choices for finding a new identity, from industrial-sized operations to artisans working from home on easily procured and increasingly refined equipment. A newcomer to the business is the Islamic State group in Syria and Iraq, which reportedly seized thousands of blank passports and equipment in towns it conquered.

French Interior Minister Bernard Cazeneuve has been pressing his European partners to do more to fight the Islamic State, which he says has created a "structure that manufactures fake documents."

That organization claimed responsibility for the November attacks that killed 130 Friday night revelers in Paris and the March attacks on a Brussels airport and subway station that killed 32 people.

Two of the suicide bombers at France's national stadium remain unidentified except for their fake Syrian passports. One of those passports had been listed by Interpol, the international police agency, as among a batch of stolen blank passports.

Germany stopped recognizing passports issued in territory controlled by the Islamic State group in Syria and Iraq at the start of the year.

Belgium raided the document den in the Brussels suburb of Saint-Gilles in October, but put out a warrant for the suspected forger Djamal Eddine Ouali only in January. On March 26, Italian police arrested the 40-year-old Algerian when he applied for a residency permit. A court in Salerno, in southern Italy, granted Belgium's extradition request on Sunday, but Ouali plans to appeal that decision, according to his lawyer, Gerardo Cembalo.

Information-sharing is critical in the effort to catch fraudsters, a message hammered out by Interpol. Its database had nearly 38 million travel documents reported lost or stolen by 166 countries as of July 2013, the latest figures available.

Layers of security are built into U.S. and European passports, the most valuable documents for anyone trying to sneak into the borderless Schengen region or to return home covertly from places like Syria. European Union passports have added value for those crisscrossing the continent, an increasingly frequent tactic, because visas aren't needed.

No passport — the ultimate identity document — is fully fraud-proof, despite holograms on identity pages, complex watermarks and biometric data contained in embedded chips.

Bruguiere, the retired anti-terrorism judge, said document forgers operate as part of the outer circle of terrorist operations — "people who are somewhat or a bit or perhaps not concerned with the operation" but who contribute logistical support such as "a hideout, for example, a car, fake papers, without being directly linked to the project."

"There are guys who meet from time to time," Bruguiere said. "And then, 'Can you give me a hand? I need fake papers.'"

___

Frances D'Emilio in Rome contributed to this report.