A request for Adil Oksuz to be extradited from Germany has been prepared following a demand from an Ankara criminal court, the Anadolu news agency said.
Berlin has never confirmed if Oksuz is in the country, but Turkey in August asked the German government to formally investigate reported sightings of him.
Ankara blames the doomed July 15, 2016, coup attempt to oust President Recep Tayyip Erdogan on US-based Islamic preacher Fethullah Gulen, and has implemented a ruthless crackdown on his suspected supporters.
But the second most wanted suspect is Oksuz, a theology lecturer whom Turkish officials accuse of being the so-called "imam" of the plot by coordinating actions on the ground in Turkey with Gulen.
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Oksuz was detained in Turkey after the coup was quashed, but was subsequently released -- allegedly because the judge was pro-Gulen -- and is now on the run.
The extradition request relates to the mass trial of 486 people, including Oksuz, over events at the Akinci air base outside Ankara, seen as the hub of the failed putsch, on the night of the coup plot.
Ties between the two countries -- key NATO allies with strong historic ties -- have nosedived over Ankara's arrest of German citizens and Berlin's strong criticism of the Turkish post-coup crackdown.