A former Turkish army chief, Gen Ismail Hakki Karadayi, has been detained over a military intervention that ousted a pro-Islamist government in 1997.
Gen Karadayi was detained at his home in Istanbul and taken to the capital, Ankara, for questioning.
He is suspected of helping what became known as the post-modern coup, as no soldiers were involved.
Former Prime Minister Necmettin Erbakan was forced to resign in 1997, being replaced by a civilian government.
Last May another six retired generals linked with Mr Erbakan's removal from power were charged.
BBC regional analyst Kumru Baser says Gen Karadayi's detention was widely expected.
The general has already given evidence to a parliamentary committee on the matter and denies that he played any part in alleged army interference in politics.
However, his deputy at the time, Gen Cevik Bir, disagrees and has insisted during all the investigation that he was simply following Gen Karadayi's orders.
Separately, investigations are continuing into other coup plots by the military.
Turkey's military has long seen itself as the guarantor of the country's secular constitution, analysts say.
It staged three coups between 1960 and 1980 and has a history of tension with the Justice and Development Party (AKP) of Prime Minister Recep Tayyip Erdogan.
The Islamist-rooted AKP, in power since 2002, emerged from Mr Erbakan's Welfare Party, which was outlawed after the army intervention.
A Turkish court sentenced three former army generals to 20 years in jail each in September for plotting another coup. More than 300 officers were convicted of involvement in the plot.
They were accused of plotting to bomb mosques and trying to trigger a war with Greece in order to justify a military coup against the elected government of Mr Erdogan in 2003.