ANKARA, Turkey—Yasar's son was only 14 when he left his home in Ankara, Turkey, to join the deadliest group in the Middle East.

Telling his father he was going with his older brother to sell vegetables in a market outside the city, he instead hitched a ride with four friends to Syria, where he was inducted into the radical militant group, the Islamic State.

When Yasar got the call from his son, he was horrified but not surprised. For a while, people in the neighborhood had been talking about children and young men traveling more than 500 miles to Syria to join the Islamic State (formerly known as ISIL or ISIS.) And his son had recently begun praying five times a day.

A socialist with little interest in religion, Yasar at first had thought that his son's display of piety was better than a life devoted to drugs which are all too common in their neighborhood.

But he hadn't realized how far things had gone.