The New York Times has gained further insight on internal structures of ISIS revealing details about its management and personnel, provided by a source that has access to Iraqi military forces and US secret services.
According to these documents the top management was directly chosen by Abu Bakr al-Baghdadi after getting to know them from US military prison Bucca about ten years ago. About 25 top level executives, called Walis, are acting partially autonomously but within a joint network of rules and communication. A third of these already served as military officers under Saddam Hussein secular regime, but became allegedly religious after 2003 when the American invasion took place.
ISIS' success seems to be owed to a hybrid structure: its commanders have traditional military capabilities trained at the Iraqi military academy as well as experience in terroristic techniques, gained during the battle against US military, and backed up by local territorial knowledge as well as widespread contacts.