The U.S. is pressing for European Islamic State militants captured in Syria and Iraq to be sent back to their home countries in an effort to ensure they don’t return to the battlefield.
One problem: Europe doesn’t want them.
As the U.S.-led coalition has rolled back Islamic State, European militants have retreated to Northern Syria, only to be captured by Kurds and other rebel forces that lack an established justice system to prosecute and imprison the fighters long-term. Their legal limbo raises the specter of militants slipping through the cracks and regrouping to carry out attacks.
But sending hundreds of militants back poses challenges. European governments could face difficulties prosecuting them given the scarcity of battlefield evidence. Washington held out the prospect of sending some prisoners to the U.S. detention center at Guantanamo Bay, Cuba, but many in Europe see the center as a symbol of U.S. detention and interrogation practices in the post-2001 era that governments have criticized.
The question of what to do with European nationals detained in territory once controlled by Islamic State loomed over a meeting of the anti-Islamic State coalition in Rome in Tuesday, and could be discussed on the margins of the Munich Security Conference beginning on Friday.
On Tuesday, U.S. Defense Secretary Jim Mattis said European countries must take responsibility for their citizens who went to Syria or Iraq to fight.
“The bottom line is we don’t want them going back on the street,” Mr. Mattis said. “We don’t want them on the street in Ankara, we don’t want them on the street in Tunis, Paris or Brussels.”
A senior defense official said dozens of militants are being detained daily in Northern Syria, and Kurdish and rebel forces are running out of room to hold them.
“Doing nothing is not an option,” Mr. Mattis said.
European capitals have publicly said they are willing to repatriate some of their nationals, such as women and children who jihadists brought to the conflict zone. Europeans who make it over the border to Turkey are subject to extradition and prosecution at home. The German government says its citizens “always have the right” to return.
Privately, however, European officials say they are loath to bring back the detainees.
“My overall opinion is no one really would like them back,” said Pieter Van Ostaeyen, a terrorism analyst in Belgium.
In addition to scarce evidence, the unwillingness of intelligence agencies to allow their information and sources to be exposed in a courtroom could make prosecutions a challenge.
Even if prosecutors win convictions, sentences in Europe are often much shorter than in the U.S., meaning national governments could face the prospect of hardened jihadists being released after their sentences.
After British nationals Alexanda Kotey and El Shafee Elsheikh were captured by Kurdish fighters in Syria, U.K. Defense Secretary Gavin Williamson said they shouldn’t be brought back to face trial and that they should never set foot in Britain again. Of the 850 Brits who have gone to Syria and Iraq to join Islamic State, some 360 have returned, according to U.K. officials.
As part of the discussions over the fate of the U.K. prisoners, Washington has held out the prospect of sending them to the Guantanamo Bay detention center, but British officials oppose that outcome.
“While we have a preference that the U.K. take them back, Gitmo remains an option,” said Steve Goldstein, the State Department’s under secretary for public diplomacy and public affairs. “We are still work at an acceptable resolution.”
Mr. Mattis declined to answer directly Tuesday when asked about Guantanamo or whether the U.S. would take some of the new prisoners.
“We want to make certain that foreign fighters are taken off the battlefield and they don’t show up somewhere else and I just want to hold at that right now,” he said.
European officials say they are focused on tracking down their nationals in Syria and Iraq and leaving them in the hands of local authorities.
France, the biggest European source of foreign fighters in territory once held by Islamic State, was among the first countries to take a hard line. Last year Paris deployed special forces to Mosul to enlist Iraqi forces in hunting and killing French nationals who joined the senior ranks of Islamic State, according to Iraqi officers and current and former French officials.
Paris isn’t seeking extradition for Frenchmen captured in Iraq, officials said, because these people should face justice in the country where they are alleged combatants.
In Syria, which has no diplomatic ties with France, French officials are working with the U.S.-backed Syrian Democratic Forces—a militia that has taken back territory from Islamic State in Syria along the border with Turkey—and other rebel forces to arrange local trials that French officials say will provide due process.
About a hundred French nationals are currently detained in Northern Syria, the government says, while the whereabouts of many others are unknown. Among the French nationals still at large is Hayat Boumeddiene, who French authorities suspect fled to Syria after her partner, Amedy Coulibaly, gunned down shoppers in a kosher supermarket in Paris days after the Jan. 2015 Charlie Hebdo attack.
In Belgium, which has been a hotbed of jihadist activity in recent years, an official said there is no legal basis for bringing Belgian fighters back for trial, but the government is continuing to examine the issue. An estimated two to three dozen Belgian foreign fighters are in detention in Syria and Iraq, another Belgian official said.
Some European officials caution that not all detainees should be barred from returning. German security officials say only a very small number of high-ranking Islamic State fighters are German, and the vast majority of them were involved in administrative work.
The militants’ families pose another set of challenges. Women who accompanied their husbands to Islamic State territory or got married there often claim they never fought, making them harder to prosecute in some European countries, according to German officials. Germany’s prosecutor general recently said the mere act of traveling to these areas should be criminalized—a step that France has already taken.
Children born in or taken to Syria and Iraq are being handled on a case-by-case basis, European officials said. While young children are likely to return home, Islamic State put many boys older than 10 through combat training and indoctrination while teenagers were deployed to fight, officials said.
Foreign Minister Jean-Yves Le Drian said in a recent interview that six French families were currently in custody in Iraq, and that he expected only the children to return to France.
Asked if he was worried Iraqi courts would impose the death penalty, which has been banned for decades in France, Mr. Le Drian responded: “They didn’t go to Iraq for tourism. They went to fight in the ranks of Islamic State.”
—Andrea Thomas in Berlin, Jenny Gross in London and Felicia Schwartz in Amman contributed to this article.