Federal prosecutors said the 36-year-old, identified only as Thomas K. in line with German privacy rules, was arrested on Wednesday on his arrival at Frankfurt airport after Afghan authorities handed him over.
He is suspected of membership in a foreign terrorist organization, according to Fox News.
Prosecutors said Thursday that the suspect traveled to Pakistan in 2012 and within a few months joined the Taliban in that country’s North Waziristan region, on the Afghan border. They said that, by March 2014, he was fighting for the group in Afghanistan.
Afghan authorities arrested the suspect in southern Helmand province in late February.