Talk to Germans of Turkish descent and many recall childhood friends who gravitated towards religious conservatism and, having donned headscarves and more modest customs, slowly and quietly drifted away. "Gulen", they say, and shrug.
They're referring to the religious movement of the US-based Turkish Sunni cleric Fetullah Gulen, built around the notion of Hizmet, or "service", which runs hundreds of educational establishments across the world, spanning from Afghanistan to Tanzania and the United States.
It's a secretive movement which originated in Turkey, whose structures remain largely opaque and who members are unwilling to profess their adherence to Gulen. This has resulted in criticism that the movement lacks transparency and has an agenda of indoctrination and Islamization through its network of schools and free tuition centres.