The discovery of a terror plot aimed at European cities underscores the growing danger posed by militants equipped with Western passports, a coveted weapon in Al-Qaeda's arsenal.
US and European security services are struggling to track young Western nationals who travel to Pakistan and elsewhere to receive training from Al-Qaeda and its allies and then return to try to carry out attacks, analysts and officials say.
"All the evidence indicates that it's an increasing threat," Arturo Munoz, a retired veteran of the Central Intelligence Agency, told AFP. "I don't think there's any doubt about that."
A new report by American and Swedish researchers out Friday warned that Western intelligence services' knowledge of the threat may "only touch the tip of a much larger, undocumented and undetected problem."
Militants with Western passports and no criminal records, or a so-called "clean skin," can move about without drawing attention and evade authorities.
"These fighters' familiarity with the targets they select adds to these individuals' capacity to cause harm," said the report by the Homeland Security Policy Institute at George Washington University.
European security officials in recent days said Al-Qaeda militants were plotting to stage simultaneous strikes in London and cities in France and Germany, with information on the plan possibly coming from a German citizen now detained in Afghanistan.
Ahmed Sidiqui, of Hamburg, reportedly was part of the Islamic Movement of Uzbekistan and told American interrogators that some operatives in the plot may already be in Europe.
"One country facing a huge concern right now is Germany," said Magnus Ranstorp from the Swedish National Defence College, one of the authors of the study.
According to Germany's Federal Office for the Protection of the Constitution, some 200 Germans or foreigners living in Germany have spent time in Pakistan with the intention of receiving paramilitary training by Islamist groups.
Authorities have concrete evidence that 65 of them underwent such training, it said.
Ranstorp told a Washington conference that Denmark also faced a serious security threat from "Western foreign fighters," with militants targeting the Danish newspaper Jyllands-Posten, which had published controversial cartoons of the prophet Mohammed several years ago. The "system was blinking red" in Denmark, he said, and even Sweden had raised its threat level this week, which he said illustrated that no country in Europe was immune from terrorist dangers.