U.S.-based Turkish cleric Fethullah Gulen at his home in Saylorsburg, Pennsylvania, U.S. July 10, 2017. REUTERS/Charles Mostoller

ANKARA (Reuters) - Turkey charged 28 people on Friday in relation to the 2016 assassination of the Russian ambassador to Ankara, naming the U.S.-based Islamic cleric Fethullah Gulen as the prime suspect in the case, the state-owned Anadolu news agency said.

Andrei Karlov was shot dead by an off-duty policeman while speaking at an Ankara exhibit opening in December 2016. The gunman shouted “Allahu Akbar” and “Don’t forget Aleppo!” as he opened fire, apparently referring to Russia’s involvement in Syria. He was shot dead by police at the scene.

President Tayyip Erdogan has said Gulen’s movement was behind the assassination, a charge the cleric has denied. Erdogan also blames the preacher’s network for an attempted military coup in July 2016.

Gulen, who has lived in self-imposed exile in the United States since 1999, has condemned the coup and denied any involvement with it.

Authorities charged Gulen and 27 others of attempting to “overthrow the constitutional order”, “being a member of a terrorist organization” and of premeditated murder, Anadolu said.

Prosecutors say the Gulen’s organization was attempting to derail relations between Turkey and Russia with the killing. At the time of the December 2016 killing of Karlov, ties between the two countries had already been strained, after Turkey downed a Russian war plane over Syria a year earlier.

Since Karlov’s assassination, ties between Ankara and Moscow have made steady improvement.