When Imam Shawan Kurdi disappeared in November, residents of his Daretu neighborhood of Erbil thought it was strange. The Friday before, Imam Shawan led prayers at the tiny Jaffer mosque but the next day, he, his wife and two daughters had vanished. Three days later, Kurdish intelligence confirmed that Imam Shawan had taken his family and two other young men from Daretu to join the Islamic State in Iraq and Syria (ISIS). He is now in Mosul, the largest city under the control of ISIS.

Imam Shawan is among hundreds of mostly young Kurds to cross the frontlines and join ISIS, who have been in battle with Kurdish forces since June. The Kurds are losing young men to the extremists and in Erbil many talk about fathers and sons fighting on opposite sides. Like ISIS, Kurds are Sunni Muslims, but nationalism has long trumped religious identity here. Residents in Daretu aren’t keen to talk about how they lost one of their own to the militants, but when pressed, they say there were signs Imam Shawan was leaning to toward ISIS.

“He always talked about…how we had to do things the way they did in the time of the Prophet Mohammed,” says Mohammed Jamil, a 54-year-old resident of Daretu who prayed behind the Imam for a year.

Imam Shawan was paid by the Kurdistan Regional Government (KRG), the body that runs the semi-autonomous Kurdish territory of northern Iraq. Officials here began to worry their youth were being radicalized in Kurdish government-funded mosques and religious schools.

“One of my employees, his son joined ISIS, and he was a student in one of our schools,” says Salam Sidan, an advisor at the KRG’s Ministry of Endowments and Religious Affairs. Late last year Sidan’s ministry fired 80 government-paid religious teachers they felt weren’t “modern” enough for the job. “Some of the Imams and Mullahs were preaching that non-Muslims are infidels,” says Sidan. “It’s natural, in a place where you have 5 or 6 million people, you are going to have some voices calling the others infidels.”

Sidan stresses that the ministry is being diligent, ensuring new hires fit with the Kurds’ ideas of acceptance and tolerance. The ministry has no exact figures on how many Kurds have joined ISIS but Sidan’s estimates around one in 5,000, which would put the number as high as 1,000. Most of these men simply disappear, raising suspicions of family and neighbors. Some are never heard from again but others surface online. “You can’t hide anything with social media these days,” says Sidan. “Everything is on Facebook. And they post proudly that they joined ISIS.”

Some appear on social media encouraging their brethren to come join ISIS and threatening the Kurdish forces known as peshmerga. In one video, titled “A message to the Peshmerga,” a man who identifies himself as a Kurdish ISIS fighter promises attacks his own people. “By sword we will chop your heads and by Quran we will implement God’s rule, Sharia,” said the young man, standing with a motley crew of militants, a black ISIS flag flapping in the corner of the screen. He warns Kurds to stop supporting the “Crusader Coalition” referring to the US-led alliance that has been targeting ISIS in both Syria and Iraq.

The Kurdish ISIS fighters pose a serious threat to the security of the KRG. Kurdish speakers usually cruise through security checks with few questions and little searching. In November, a truck exploded in front of government buildings in Erbil. An ISIS-affiliated website posted a statement claiming the attack and saying it was carried out by a Kurdish recruit they called Abdel Rahman el-Kurdi and two others. “They were able to cross all these borders,” reads the statement. “And hit the heart of Erbil….We ask god to accept them as martyrs.”

But here they will not be accepted. Most families are ashamed that their kin have pointed weapons at fellow Kurds and intelligence officials have said those killed fighting with ISIS will not be allowed funerals in Kurdish territory.

Salar Salim contributed to this report

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