Tarik S. entered Syria in the winter of 2013 and had joined the Isis organization within weeks, federal prosecutors say.
Investigators further claim he took part in fighting and appeared in propaganda videos.
Police arrested Tarik S. on Wednesday and he has already appeared in front of federal judges in Karlsruhe.
In one of the propaganda films Tarik S. crouched in a hut and talked about the war, Spiegel reports.
“A rocket was just fired at us” he claims in the video. “We’re not sure where it hit, if the roof collapses on us we’re done for.”
In another video he called on fellow believers in Germany to “come to the holy land.”
Research by Spiegel found that after he left school Tarik S. failed to find a job.
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He only occasionally made use of an organization in his native Bielefeld that helps young people get practical qualifications, and constantly broke off internships after a few weeks.
Broadcaster WDR writes that he was radicalised when he travelled to Egypt in 2013 to join supporters of the Islamist President Mohammed Morsi, who had been overthrown by the military.
During protests in Cairo he was shot at and when he came back to Germany he joined an Islamist group, traveling to Syria shortly afterwards.