Fethullah Gulen is a Turkish preacher, writer, and a political figure.

Russian investigators arrived in Turkey on Tuesday to probe the assassination of Moscow's ambassador as the Turkish government pointed the finger of blame at exiled cleric Fethullah Gulen.Veteran diplomat Andrei Karlov was shot nine times in the back by off-duty Turkish policeman Mevlut Mert Altintas at the art gallery opening of a show of Russian photography on Monday.The brazen killing stunned Ankara and Moscow, which have rowed repeatedly over the Syria conflict but had recently begun to cooperate closely on the evacuations from war-wrecked Aleppo.An unprecedented three-way meeting on Syria between the foreign ministers of Turkey, Russia and Iran went ahead in Moscow Tuesday despite the assassination, with the diplomats backing a widening of a truce.Turkish Foreign Minister Mevlut Cavusoglu, meanwhile, pinned the assassination on Gulen's group, which Ankara says also orchestrated an attempted coup in July, the state-run Anadolu news agency said.In a conversation with US Secretary of State John Kerry, Cavusoglu said "Turkey and Russia know that behind the attack... there is FETO," it said, referring to Turkey's acronym for Gulen's organisation.The US-based Gulen had earlier issued a statement to condemn the assassination as a "terrorist act" that left him "shocked and deeply saddened."Six people have been detained over the Karlov assassination, including the sister, mother, father and uncle of the 22-year-old Altintas, Turkish media said.Adding to the jitters, with Turkey already on high alert after a string of deadly attacks, an individual also fired outside the US embassy in Ankara overnight.The mission said in a statement that no one was hurt and the individual was detained but the embassy and consulates in Istanbul and Adana were closed for normal operations.Russian President Vladimir Putin ordered his secret services to boost security at home and abroad, and to step up cooperation with foreign intelligence services.A Russian investigative team visited the scene of the attack at the Contemporary Arts Centre in central Ankara as part of a joint probe with Turkey.Erdogan said in a speech in Istanbul that he and Putin agreed in a phone call after the murder that "our expanding areas of cooperation with Russia, particularly on Syria, will not be hampered by this attack".Dramatic footage of Monday's assassination showed Karlov stumble and crash to the ground on his back as Altintas brandished his automatic pistol at terrified onlookers who cowered behind cocktail tables.The lone gunman shouted "Allahu Akbar" ("God is greatest") and "Don't forget Aleppo", vowing that those responsible for events in Syria would be held accountable.Altintas did not go through the metal detector security check when he entered the exhibition and was warned by a security officer, according to the Cankaya municipality where the exhibition centre is located.But after showing his police ID, he was allowed to proceed, it said.The Hurriyet daily said Altintas, who had worked for Ankara's anti-riot police for the last two-and-a-half years, had stayed at a nearby hotel to prepare for the attack, shaving and putting on a smart suit.He was killed by police after a 15-minute standoff.Ankara mayor Melih Gokcek, known for his outspoken comments, became the first senior official to link the killing with Gulen's group.His suggestion was also repeated in the pro-government press, before being amplified in the Anadolu report quoting Cavusoglu.US President-elect Donald Trump had on Monday condemned the envoy's assassination, calling the gunman a "radical Islamic terrorist".A Russian plane carrying the ambassador's body landed in Moscow, where it was met by Cavusoglu and Russian counterpart Sergei Lavrov.Turkey gave the murdered envoy a full honour guard on his departure from Ankara, as an Orthodox Russian priest, watched by Karlov's widow, read the last rites and swung incense over the coffin.Cavusoglu announced in Moscow that the street where the embassy is located would be named after the 62-year-old envoy, a career diplomat who had notably served as ambassador to North Korea.The killing came after days of protests in Turkey over Russia's role in Syria, although Moscow and Ankara are now working closely together to evacuate citizens from the battered city of Aleppo.Turkey and Russia stand on opposite sides of the conflict, with Ankara backing rebels trying to topple Moscow's ally President Bashar al-Assad.

But the rhetoric has warmed considerably since a reconciliation deal was signed earlier this year and the tripartite meeting Tuesday was just the latest in a series of contacts.Russia, Iran and Turkey agreed on Tuesday to guarantee Syria peace talks and backed expanding a ceasefire in the war-torn country, Russian Foreign Minister Sergei Lavrov said after the meeting.