Davut (Halis) Hancı, described as Gülen`s right-hand man, apparently entered Turkey two days before the failed coup attempt, the presidency official told reporters.
Hancı is accused of transferring funds to Gülen.
Separately, the official confirmed that Turkey also detained the son-in-law of Akın Öztürk, a former air force chief already arrested as one of the key suspects in the attempted coup.
Lieutenant colonel Hakan Karakuş was detained in Ankara, it said.
FETÖ, also referred to as the Gülen Movement, operates on a secretive structure, with leaders and representatives at the district, provincial and nation-wide levels who both act as insiders and decision-makers steering the movement.
They are accused of being behind the July 15 coup attempt, when over 240 people were killed and over a thousand were injured by pro-coup Gülenist soldiers within the military.
Gülen is among Turkey`s most wanted, with the country exerting a tremendous effort to obtain an international arrest warrant for him.
He lives in a compound owned by his movement in Saylorsburg, Pennsylvania.
Ankara is seeking to secure his extradition from the U.S and has officially submitted the documents for extradition earlier this week.
Gülenists run a vast network of schools around the globe, but are primarily invested in charter schools in the U.S., which receive government funding but operate independent of the public school system.
Dozens of schools associated with Gülenists are facing criticism and are under investigation over the alleged misuse of federal grants and the abuse of a visa scheme being used to funnel foreign teachers into charter schools who are brought from Turkey to live in the U.S.
FETÖ, which has seen its members and sympathizers purged from state institutions, including the police and judiciary, was designated by authorities as a national threat, a classification for terrorist organizations.
Gülenists are accused of illegally wiretapping thousands of people, from the prime minister to journalists and other prominent figures.
They are also accused of imprisoning critics or anyone seen as an obstacle to the movement`s attempts to gain further clout through sham trials.
Hundreds of generals, academics and others were detained for years in cases in which they were accused of attempting to stage coups.
It was later revealed that they were detained on charges based on false evidence planted by Gülenist members of law enforcement.
Fethullah Gülen`s key aide captured after failed coup attempt
24 July 2016
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Turkey has captured a key aide to the Fethullah Gülen, the leader of Gülenist Terror Organization (FETÖ) in northeastern Trabzon province, a presidency official said on Saturday.
