A Washington Post investigation, however, later found that Sarfo was much more involved in executions than he wanted to make the public and investigators believe. The revelations underlined questions about whether returnees can be trusted as informants on other militants, for instance.

A confidential new report by German authorities now provides deep insights into what investigators know about the mindsets of those who have returned to Europe. At the core of the report, partially published by German newspaper Die Welt, is the question of what differentiates a disillusioned returnee from a potentially dangerous individual who still believes Islamic State ideology and might be willing to conduct attacks in Europe.

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Only 10 percent of all returnees are disillusioned with the ideology, the new data suggest. Eight percent of them might only have come back to their home country Germany to recover from being on the battlefield, before trying to return to Syria and Iraq, the report says.

According to the report, about half — 48 percent — stay committed to extremist ideology and remain friends with other extremists. The latter number might be particularly worrisome to experts: Most of those who left for Syria or Iraq were radicalized by friends or individuals they met in mosques or in Islamic seminars, the study says.

The large number of returnees since 2013 has put pressure on European authorities who lack the needed number of officers to monitor terrorism suspects 24 hours a day. The lack of observation could turn out to be a dangerous gamble, the report suggests.

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In total, 784 Germans who joined militant groups in Iraq and Syria were analyzed. Their number peaked in the third quarter of 2014, but has since dropped significantly. Only a handful of Germans are believed to have traveled to Syria and Iraq over the past couple of months to join the Islamic State, investigators believe.

The same is expected to be the case in other E.U. nations. With the Islamic State increasingly under pressure in Syria and Iraq and border controls now more effective, the willingness of Western recruits to join the group has dropped.

Two hundred and seventy-four Germans are known to have returned to their home country. The number of monthly returnees spiked in 2013 and 2014, indicating that those who were disillusioned by the Islamic State decided to leave the group quickly.

The Islamic State has since turned more repressive toward its own members, executing European recruits for attempting to flee Syria and Iraq.