Turkish prosecutors have issued detention warrants for 19 Gülenists -14 of whom are police officers- in connection with the disappearance of an oil official 17 years ago.
Istanbul Chief Public Prosecutor's Office called for the detentions on Sunday in the case of Cevzet Soysal, an official at Turkey's national oil company who has been missing since 1998.
Soysal was working for the Turkish Petroleum Corporation when he went missing in the eastern province of Batman. Although he has never been discovered, a book published earlier this year claims that Soysal was abducted and killed.
A former police intelligence officer, Sabri Uzun, claimed in his book - called "İn" ('Cave' in English) that Soysal had met with foul play.
Soysals' wife, Kaside Soysal, told Anadolu Agency in January that his family believes he is merely missing.
An investigation by the Istanbul Chief Public Prosecutor's Office now claims that the incident is "linked to the 'parallel structure'".
Turkish police are searching for the suspects.
The ongoing 'parallel state' probe has seen hundreds of arrests since last summer for alleged links to the U.S.-based cleric Fetullah Gülen and his so-called Gülen movement.
The alleged network is accused of orchestrating a corruption probe which targeted high-ranking Turkish officials in December 2013 in a bid to overthrow the government.
According to the Turkish government, the 'parallel state' or 'parallel structure' refers to a purported group of Turkish bureaucrats and senior officials embedded in the country's institutions, including the judiciary and police, who have been allegedly trying to undermine the Turkish government.