In the northern Iraqi city of Halabja, near the border with Iran, we knock on the door of a 16-year-old boy who disappeared. His family says he lied to them, saying he was going on a picnic with a teenage friend. But they never came home.
"He disappeared in May," says the boy's older sister. "A few days later a letter arrived in his handwriting. It said, 'I'm in Syria. Don't look for me.' "
The boy, like most everyone in this city, is a Kurd, most of whom are Sunni Muslim. He joined the so-called Islamic State, a Sunni Muslim extremist group also known as ISIS.