to be terrorist organizations.

A particular focus has been on countries like Turkey, whose porous 500-mile border has allowed thousands of militants to cross into the Syrian battlefield and into Iraq. In 2013, Turkey denied entry to 4,000 people who had been on a no-entry list and detained more than 92,000 people on its border.

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Altogether, about 18,000 foreign combatants, including 3,000 Europeans and other Westerners, have traveled to fight in the region since the Syria conflict erupted in 2011, according to American intelligence estimates. More than 500 veterans of the Syria campaign have returned to Europe, according to Richard Barrett, a former British intelligence officer who has researched the figures for the Soufan Group, a security consultancy.

In the United States, where about 150 people have tried or actually goneas a caliphate. Counterterrorism specialists have seen criminal gang members from as far as Sweden seeking adventure and violence in the fight.

Fears that former fighters may carry out attacks in their native countries have been intense since Mehdi Nemmouche, a 29-year-old French Muslim, killed four people at the Brussels Jewish Museum last May after spending a year in Syria. The French authorities knew he had gone to fight in Syria and had been told by German officials that he had returned to Europe.

France, with about five million Muslims, the largest Muslim population in Western Europe, has seen a growing number of calls for citizens to turn to jihad. More than 1,000 French citizens have left or plan to leave France to join the ranks of jihadist groups in