The leader of ISIS jihadist group and self-proclaimed “caliph” Abu Bakr al-Baghdadi, spent nearly a year in US custody in Iraq in 2004 as a “civilian detainee,” declassified military documents have revealed.

The files were obtained by Business Insider through a Freedom of Information Act request, revealing new details about the mysterious jihadist leader. The Islamic State (IS, formerly ISIS/ISIL ) chief was identified by his birth name, Ibrahim Awad Ibrahim Al Badry, in the detainee information records, viewed by the website.

The documents helped determine the time, spent by Baghdadi in US custody, more precisely as there had previously been conflicting reports on the issue.

According to the records, his “capture date” was February 4, 2004, with the detention taking place in Fallujah in central Iraq. Baghdadi was then held in several prison facilities in the country, including Camp Bucca and Camp Adder, with the date of his “release in place” being December 8, the same year.

The papers list him as a “civilian detainee,” meaning that he was not considered a member of any militant group at that time, but was still held for security reasons.

The declassified records identified Baghdadi’s “civilian occupation” as “administrative work (secretary).”

The book called “ISIS: Inside The Army of Terror” by Michael Weiss and Hassan Hassan claims that Baghdadi was arrested together with Nessayif Numan Nessayif, who was the real target of the US military.

The date of his birth was redacted in the files received by Business Insider, but the website said that the current IS leader was listed as having been 43 years old in 2014. The paper also included details on Baghdadi’s family, revealing that he was married and next of kin was an uncle. However, the names of his family members were also redacted.

The Islamic State has declared a caliphate, with Baghdadi as its ruler, after capturing large parts of Iraqi and Syrian territory last summer. The jihadist group is notorious for its brutality, ethnic cleansings of minorities and executions of Western hostages.

A US-led coalition has been conducting regular airstrikes against the Islamic State in Iraq and Syria since August 2014, with several unconfirmed reports stating that Baghdadi might have been injured in one of the raids.