Oğuz Karakuş, a retired police chief who has been held in pre-trial detention since May 2016, died on Monday following a heart attack in an İzmir prison, several twitter users claimed.
The claim has neither been confirmed nor denied by the Turkish authorities.
Karakuş was arrested by an İzmir court in May 2016 over his alleged links to the Gülen movement, which is accused of masterminding a coup attempt on July 15, 2016.
The movement denies any involvement.
At least 30 people, among whom are police officers, prosecutors and teachers, have been found dead in Turkish prisons since the failed coup attempt in 2016 to April 2017, causing serious concern about the fate of thousands of civilians who have been kept in jail in poor conditions across the country.
The Turkish government blames the Gülen movement for the abortive coup, while the latter denies any involvement. More than 160,000 people have experienced police custody over Gülen links since the summer of 2016.