Boydak and six others had been held by the police in a city-wide operation conducted in Kayseri, in Central Anatolia. Seven suspects were held in morning raids when police swooped down on their properties and working offices. Four people are still wanted in the central Turkish city in the ongoing investigation.

The suspects are accused of being members of the parallel state, led by the U.S.-based cleric Fethullah Gülen. Wednesday's raids were based on arrest warrants which had been issued for 11 Kayseri-based businessmen, as part of the parallel state investigation.

Police also raided the rectorate's office of Melikşah University, the first privately-funded university in Kayseri. Some executives were detained during the operation. The sweep follows a recent police raid that has aimed at the premises of the Koza İpek-Holding, known as a fierce critic of the government, in Ankara, earlier this month.

Boydak, who is also chairman of the board of trustees at Melikşah University, is said to be close to U.S.-based cleric Fethullah Gülen. Gülen, who has lived in self-imposed exile in Pennsylvania since 1999, is wanted for forming and running an armed terrorist group in Turkey. The one-time ally of President Recep Tayyip Erdogan has now been accused of plotting to overthrow the Turkish government, using his supporters who have held key posts in the police and judiciary. The cleric strongly denies the claim.