Following a failed coup attempt in July 2016 against the Turkish President, Recep Tayyip Erdogan, many Turkish citizens and visitors from other countries have been held as parties to this event. There was a round-up of tens of thousands of citizens who were considered parties to the failed coup.
Several American dual citizens were arrested in this round-up, one being an employee of NASA, Serkan Golge. Very little information was presented against him, but he was sentenced recently to 7.5 years in prison. Many believe that this was to serve as a bargaining chip for Erdogan to be able to barter for the return of an Imam whom Erdogan blames for the coup who is currently living in Pennsylvania. The U.S. government has not returned the Imam, Fetullah Gullen, to Turkey because extradition is a court process that requires certain procedures be followed and those are currently in process.
Other American citizens have been arrested during this time and allegedly held for the same purpose.
For opinion piece, see: Washington Examiner, The Heartbreaking Story of Turkey Sentencing An American NASA Scientist to 7.5 Years in Prison, (by Eugene Chudnovsky, February 10, 2018). http://www.washingtonexaminer.com/the-heartbreaking-story-of-turkey-sentencing-an-american-nasa-scientist-to-75-years-in-prison/article/2648716