Police in Turkey detained six people on Tuesday in connection with the shooting of Russian Ambassador Andrei Karlov on Monday night. Authorities have widened the investigation to the relatives of the assailant, an off-duty police officer identified as 22-year-old Mevlut Mert Altintas. His father, mother, sister and two other relatives, along with his flatmate in Ankara, were detained, Reuters reported citing the state-run Anadolu agency.

The incident, which took place at an art gallery in Turkish capital Ankara, was condemned by both countries as an attempt to sabotage bilateral relations, which have been deteriorating since the civil war in Syria began four years ago. Russia’s Foreign Ministry referred to the incident as a “terrorist attack”. President Vladimir Putin said, “The only answer to the murder of the Russian ambassador to Turkey must be the intensification of the struggle against terrorism.”

A senior Turkish security official told Reuters that investigators were also trying to ascertain whether Altintas had links to United States-based preacher Fethullah Gülen, whom the country believes was behind the failed military coup on July 15. An arrest warrant was issued for the cleric, who had denied responsibility for the attempted military takeover.

The had gunman stood behind Karlov while he was making his speech at an exhibition titled “Russia as seen by Turks”. Altintas was killed by the police after the attack. A video from the incident shows Karlov making a speech at the event when Altintas began to fire indiscriminately, shouting “Don’t forget Aleppo”.