Interpol urged all countries on Tuesday to obtain biometric data from fighters for the Islamic State and other extremist groups to help law enforcement track them down, especially when they return home.

Interpol Secretary General Jurgen Stock said in an interview with The Associated Press that the international police organization only has biometric data — fingerprints, DNA, iris scans and the like — for about 10 percent of the 9,000 “foreign terrorist fighters” in its database.

Stock said Interpol is helping countries to develop biometric technology not only to identify fighters from extremist groups but criminals as well.

He cited the case of “a terrorist” who attacked a police station in France last year who had traveled across Europe using 20 different identities — something that could have been thwarted with biometric data.

Stock said there are an estimated 20,000 to 30,000 “foreign terrorist fighters” from almost all over the world — and about 15,000 from over 100 countries remain mainly in Syria and Iraq.

With 9,000 names in the Interpol database, this means that about 6,000 of the extremist fighters are not on an international register where they could be tracked, which Stock said is a “serious gap.”

But he said the database has grown rapidly when it started in 2013 with just 12 files, and will hopefully continue to add names and biometric data.

The Interpol chief came to the United Nations from the organization’s headquarters in Lyon, France to speak to the General Assembly which adopted a resolution expanding the organizations cooperation with the U.N.

Stock called the Internet “a virtual university of terrorism,” where extremist groups attract and radicalize would-be fighters, and where information on building or buying bombs and explosives is readily available.

“The threat level with regard to international terrorism is unprecedented,” he said, with international “terrorists” moving to short-term actions using simpler methods such as knives, axes and in the deadly attack in Nice, France on July 15 a truck.

“We are fighting a terrorist network or an organized crime network with a law enforcement network,” he said.

But Stock said it’s “not easy” because of legal issues which differ in various countries, including on sharing information, and the difficulties in ensuring that relevant data gets to police, border guards and other officials who need it.

He said the increasing use of encrypted websites by extremist groups is also posing “a huge challenge to law enforcement” authorities trying to conduct surveillance or track fighters and potential “terrorists.”

The “dark net” has also become a major trading place for weapons and explosives used by “terrorists,” Stock said.

“Investigations into the dark net are not impossible, of course,” he said. “We are developing our tools, but it creates a challenge.”

Stock also said Interpol is cooperating with industry on new tools “to make sure that there’s no safe haven for terrorists or criminals.”

(AP)