BERLIN, April 19 (Reuters) - A former intelligence officer for the late Iraqi dictator Saddam Hussein was the mastermind behind Islamic State's takeover of northern Syria, according to a report by Der Spiegel that is based on documents uncovered by the German magazine. Spiegel, in a lengthy story published at the weekend and entitled "Secret Files Reveal the Structure of Islamic State," says it gained access to 31 pages of handwritten charts, lists and schedules which amount to a blueprint for the establishment of a caliphate in Syria.