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By The Associated Press

ANKARA, Turkey — The Turkish policeman who assassinated Russia's ambassador 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 on Tuesday inspected the art gallery in the Turkish capital of Ankara where Ambassador Andrey Karlov was shot dead Monday evening by Mevlut Mert Altintas. The 22-year-old gunman, a member of Ankara's riot police squad, had shouted slogans about the embattled Syrian city of Aleppo as he killed the envoy.

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Russia's entrance into Syria's war helped turn the tide of the conflict and heralded a series of victories for government forces. Up until a few months ago, the Russian military was bombing rebel positions in Aleppo.

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 to The Associated Press 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.

Still, Turkish police have detained seven people in connection with the gunman: his parents, sister, three other relatives and his roommate in Ankara, Turkey's state-run Anadolu news agency reported.

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

Andrey Karlov, the Russian Ambassador to Turkey, speaks at a photo exhibition in Ankara moments before a gunman (seen rear on the left) opened fire on him on Monday.Burhan Ozbilici / AP

Karlov's body was flown home to Moscow on Tuesday afternoon after an emotional ceremony at Ankara Airport attended by Turkish government officials and diplomats. Karlov's wife, Marina Karlova, wept as her husband's flag-draped coffin was carried by a Turkish honor guard. She laid two red carnations on the coffin before it was loaded onto the aircraft.

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"Ambassador Karlov has become the eternal symbol of Turkish-Russian friendship," Deputy Prime Minister Tugrul Turkes said at the ceremony.

Turkey and Russia, which have backed opposing sides in the Syrian war, vowed not to let the killing disrupt efforts to repair their ties.

Turkish President Recep Tayyip Erdogan said he and Russian President Vladimir Putin, with whom he spoke by phone Monday night, were "in agreement that this was an open provocation."

A joint investigation was being conducted into the ambassador's death, he said. "Together with Mr. Putin, we have an understanding that our cooperation, especially in Syria, will not be affected by this attack."