The Russian envoy was delivering a speech at the Contemporary Arts Gallery in the Cankaya district when an assailant opened fire on the diplomat before shooting into the air.
The perpetrator, who was wearing a suit and a tie, shouted "Allahu Akbar" ('God is great' in Arabic) during the attack, AP reports, citing their own photographer.
Karlov was killed during an appearance at the Museum of Modern Art in Ankara, the capital. He was also heard shouting in Turkish, "Don't forget Aleppo!". "Unless our towns are secure, you won't enjoy security. Everyone who is involved in this suffering will pay a price". The Turkish government is a fierce opponent of Syrian President Bashar al-Assad while Russian Federation supports him. The country's involvement in Syria has sparked protests in Istanbul, including in front of the Russian consulate there.
"Both Turkey and Russia know the Gulenist Terror Cult was behind Russian envoy Andrey Karlov's assassination", sources quoted Cavusoglu as telling Kerry. "It was determined that the people with whom he graduated from school were from a FETO team", the senior security official said of the attacker.
US officials are pushing back against claims that a political movement supported by a Turkish cleric living in Pennsylvania is behind the assassination of Russia's ambassador to Turkey.
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The Kremlin has refused to speculate whether Altinas was operating as a lone wolf or as part of a bigger operation, but said the murder was a blow to Turkey's prestige. The killing has been seen as an attempt at revenge in response to Russia's support of Syrian President Bashar al-Assad regime's acts of violence against civilians in eastern Aleppo which has led to mass evacuations.
Russian Federation and Turkey have been involved in conflict in Syria across the border from where over 2 million Syrian refugees have settled.
Mr Karlov's body is being flown back to Russia and on Tuesday afternoon his coffin was carried across Esenboga airport's tarmac, draped in a Russian flag.
The Ankara mayor said on Twitter the gunman as a 22-year-old police officer - now known to be Mevlüt Mert Altıntaş - who was off-duty at the time. CNN Turk TV reported that police had detained Mevlut's sister and mother.