An off-duty police officer in civilian clothing, identified as 22-year-old Mevlut Mert Altintas, shot down Andrey Karlov, the Russian envoy, as he was inaugurating a photo exhibition in the Turkish capital, Ankara, on Monday.

On Tuesday, the state-run Anadolu agency said the attacker's mother, father, sister and two other relatives were being held in the western province of Aydin, while his flatmate in Ankara had also been detained.

Kremlin spokesman Dmitry Peskov also said on Tuesday that Russian investigators had flown to Turkey earlier in the day to help probe the killing as per a mutual decision taken by Russian President Vladimir Putin and Turkish President Recep Tayyip Erdogan.

"Eighteen people will work in the group. The group will act in Turkey within the framework of the investigation into the murder," Peskov said.