Hearings were set to open in the criminal trial of a group of people accused of involvement in the 2016 slaying of Russia's ambassador to Turkey.

Turkish authorities have indicted 28 people of involvement in the killing of Andrei Karlov, who was shot by an off-duty police officer at the opening of a photography exhibition in the Turkish capital Ankara in December 2016.

It wasn't immediately clear how many of the alleged suspects would be appearing when the January 8 trial begins in Ankara.

Turkish and Russian news reports have said that eight people were in police custody including four former police officers and the organizer of the exhibition where Karlov was killed.

The gunman was killed by police shortly after the diplomat's killing.

Officials have alleged that the gunman belonged to a network led by a U.S.-based Muslim cleric named Fethullah Gulen, and that Karlov's killing was aimed at derailing relations between Turkey and Russia.

Gulen's movement has been accused of masterminding a failed coup earlier in 2016.

Turkey has repeatedly sought Gulen's extradition from the United States, but Washington has resisted the pleas.

With reporting by AFP, Anadolu