The assailant who killed Russia's ambassador to Ankara was unlikely to have acted alone, a senior Turkish government official said Tuesday, as investigators from both countries hunted for clues as to who might have been behind the killing.

Russian investigators arrived in the Turkish capital of Ankara on Tuesday morning and headed to the art gallery where Ambassador Andrei Karlov was shot dead Monday evening by Mevlüt Mert Altıntaş. The 22-year-old gunman, a member in Ankara's riot police squad, shouted slogans about the embattled Syrian city of Aleppo as he killed the envoy.

The senior government official described the killing as "fully professional, not a one-man action" and said the attack was well-planned. The official spoke on condition of anonymity because he was not authorized to release details to the press.

Turkish authorities have not publicly released any information on the investigation or on a possible motive for the policeman.

According to Anadolu Agency, Altıntaş took leave from work and on Dec. 14 made a hotel reservation near the exhibition center. He arrived at the hotel on Monday. Police searched his hotel room, which was later sealed.

Foreign Minister Mevlüt Çavuşoğlu, speaking at a previously scheduled meeting on Syria in Moscow with Russian Foreign Minister Sergey Lavrov and their Iranian counterpart, said Turkey and Russia would work together to determine who was behind the "heinous terror attack."

"Turkey and Russia have shown the world what they can achieve when they cooperate," Çavuşoğlu said, referring to the cease-fire deal that paved the way for the evacuation of thousands of people from east Aleppo.

Both foreign ministers lay flowers in front of a photograph of the ambassador at the Russian Foreign Ministry mansion where talks were taking place. Çavuşoğlu said a street where the Russian Embassy in Ankara is located would be renamed for Karlov.