BERLIN (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.

Iraqi security forces ride a vehicle past a wall painted with the black flag commonly used by Islamic State militants, near former Iraqi president Saddam Hussein's palace in Tikrit April 1, 2015. REUTERS/Thaier Al-Sudani

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.

The documents were the work of a man identified by the magazine as Samir Abd Muhammad al-Khlifawi, a former colonel in the intelligence service of Saddam Hussein’s air defense force, who went by the pseudonym Haji Bakr.

Spiegel says the files suggest that the takeover of northern Syria was part of a meticulous plan overseen by Haji Bakr using techniques -- including surveillance, espionage, murder and kidnapping -- honed in the security apparatus of Saddam Hussein.

The Iraqi national was reportedly killed in a firefight with Syrian rebels in January 2014, but not before he had helped secure swathes of Syria, which in turn strengthened Islamic State’s position in neighboring Iraq.

“What Bakr put on paper, page by page, with carefully outlined boxes for individual responsibilities, was nothing less than a blueprint for a takeover,” the story by Spiegel reporter Christoph Reuter says.

“It was not a manifesto of faith, but a technically precise plan for an ‘Islamic Intelligence State’ -- a caliphate run by an organization that resembled East Germany’s notorious Stasi domestic intelligence agency.”

The story describes Bakr as being “bitter and unemployed” after U.S. authorities in Iraq disbanded the army by decree in 2003. Between 2006 to 2008 he was reportedly in U.S. detention facilities, including Abu Ghraib prison.

In 2010 however, it was Bakr and a small group of former Iraqi intelligence officers who made Abu Bakr al-Baghdadi the official leader of Islamic State, with the goal of giving the group a “religious face”, the story says.

Two years later, the magazine says, Bakr traveled to northern Syria to oversee his takeover plan, choosing to launch it with a collection of foreign fighters that included novice militants from Saudi Arabia, Tunisia and Europe alongside battle-tested Chechens and Uzbeks.

Iraqi journalist Hisham al-Hashimi, whose cousin served with Bakr, describes the former officer as a nationalist rather than an Islamist. The story argues that the secret to Islamic State’s success lies in its combination of opposites - the fanatical beliefs of one group and the strategic calculations of another, led by Bakr.

Spiegel said it had obtained the papers after lengthy negotiations with rebels in the Syrian city of Aleppo, who had seized them when Islamic State was forced to abandon its headquarters there in early 2014.