Prosecutors in Turkey issued arrest warrants for 63 active-duty military personnel Friday over links to a group accused of attempting a coup in 2016.

Istanbul Chief Public Prosecutor’s Office said the suspects included four colonels and came from the army, navy, air force and gendarmerie. Early morning raids across the country resulted in 56 suspects being detained.

They are allegedly tied to an outlawed group that Turkey refers to as the Fethullahist Terror Organization, or FETO. Its leader, Fethullah Gulen, died in October last year in the United States, where he had lived since 1999 in self-imposed exile.

Some 290 people were killed in July 2016 when rogue military units took to the streets of Ankara and Istanbul in a bid to depose the government of President Recep Tayyip Erdogan. Jet fighters bombed the parliament building and presidential palace while Erdogan narrowly escaped assassination or capture while vacationing on the west coast. 

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A subsequent purge of the military, police, judiciary and other state agencies saw tens of thousands arrested. Schools, businesses and media organizations tied to Gulen were closed down.

The prosecutor’s statement said those targeted Friday were identified through telephone communications and said FETO still posed the “greatest threat to the constitutional order and survival of the state.” Since the failed coup, 25,801 military suspects have been detained, it added.

The statement did not specify the exact charges against the suspects.

Gulen, a former cleric, amassed a worldwide following over decades and aided Erdogan’s rise to power in 2003. The alliance broke down after the government closed some Gulen-run educational establishments and Gulenists in the police and judiciary pursued corruption allegations against Erdogan’s government.

Gulen always denied any involvement in the failed coup. He was wanted in Turkey, which repeatedly demanded his extradition from the U.S. 

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The coup attempt contributed to the acceleration of authoritarian tendencies in Turkey, with Erdogan’s government implementing measures that consolidated his powers.