A former Turkish cop assassinated Russia’s ambassador to Turkey on Monday – shouting “Allahu Akbar!” and vowing revenge for Aleppo as he fired five bullets into the diplomat’s back during an art exhibit opening in Ankara.

Mevlüt Mert Altıntaş, a former member of the Turkish capital’s riot police, was not on duty when he stood behind Ambassador Andrey Karlov as he gave a speech at an art gallery, the Russian Sputnik news agency reported.

Altıntaş, who managed to enter the event using a police ID while wearing a suit and tie, was seen in footage released by Dutch news site nos.nl shouting, “Don’t forget Aleppo! Don’t forget Syria! As long as our brothers are not safe, you will not enjoy safety!

“Whoever has a share in this oppression will pay for it one-by-one,” he added. “Only death will take me away from here.”

The gunman was shot dead shortly after the attack, which wounded three other people.

The attack comes a day before a meeting of Russian, Turkish and Iranian foreign and defense ministers in Moscow to discuss Syria.

Russia and Iran have backed Syrian President Bashar Assad throughout the nearly six-year conflict, while Turkey has supported Assad’s foes. U.S. State Department spokesman John Kirby said U.S. officials were aware of reports about the shooting.

A top ally of Russian strongman Vladimir Putin said a NATO country was “highly likely” to have been behind the shooting.

Frantz Klintsevich, deputy chairman of the Russian upper chamber’s defense and security committee, called the assassination “a planned action,” the Daily Mail reported.

“Everyone knew that he was going to attend this photo exhibition. It can be ISIS, or the Kurdish army, which tries to hurt (Turkish President Recep Tayyip Erdoğan,” he said.

“But may be – and it is highly likely – that representatives of foreign NATO secret services are behind it. What has happened is a true provocation, a challenge. It is a challenge for Russia,” said Klintsevich, a member of the ruling council of United Russia party, Putin’s vehicle of power.

Foreign Ministry spokeswoman Maria Zakharova said: “We call this event an act of terror. Terrorism will not win. We will strongly fight it.”

Karlov was killed on the eve of a key meeting between the Russian, Turkish and Iranian foreign ministers about the war in Syria, the Russian RIA news agency reported.

Photos released by the Turkish daily Hurriyet show a man wearing a black suit and holding a pistol standing next to a body lying on the floor.

“When the ambassador was delivering a speech, a tall man wearing a suit fired into the air first and then took aim at the ambassador,” Hasim Kilic, a correspondent for Hurriyet, told AFP.

A private guard does not accompany Karlov during public events, Hurriyet reported.

Other people at the exhibit fled for their lives after the gunman opened fire, he added.

Special forces surrounded the building amid the sound of gunfire, witnessed told the paper.

Moscow was in touch with Turkish authorities about the attack, Zakharova said, Reuters reported.

“Today, during a public event, an unknown person opened fire chaotically,” Zakharova said. “As a result, Andrey Karlov, the ambassador to Turkey, received a gunshot wound.”

Putin was informed about the attack and will be studying a report from the intelligence services and Foreign Ministry on the incident, Kremlin spokesman Dmitry Peskov told the Interfax news agency.

Interior Minister Süleyman Soylu rushed to the scene, an area where most foreign embassies are located, including Russia’s mission.

Ankara Mayor Mayor Ibrahim Melih Gökçek said Altıntaş’ motives likely had more to do with his following of Turkish preacher and former imam Fethullah Gülen and not Syria, Reuters reported.

Gülen fled Turkey in 1999 and has lived in self-imposed exile in the Poconos.

Gökçek said Altıntaş may have been a member of the Haberturk terror group and had ties to Gülen’s Islamist transnational religious and social movement.

Turkish President Recep Tayyip Erdoğan has blamed Gülen for orchestrating the July coup attempt and has demanded that the US extradite him to Turkey.

“This man is the leader of a terrorist organization that has bombed my parliament,” Erdoğan said in an interview on “60 Minutes,” the International Business Times reported.

“We have extradited terrorists to the United States in the past, and we expect the same thing to be done by the United States,” he said.

Gülen has denied involvement in the uprising, which killed more than 250 people.

The attack came after days of protests in Turkey over Russia’s role in Syria, although Moscow and Ankara are cooperating to evacuate citizens from war-torn Aleppo.

Protesters in Turkey have held Moscow responsible for human rights violations in Aleppo, where thousands of trapped civilians and rebel fighters have been awaiting evacuation.

The planned evacuations from eastern Aleppo were part of a wider Russian- and Turkish-brokered deal that would simultaneously allow more than 2,000 sick and wounded people to leave two pro-government villages that have been besieged by Ankara-backed rebels.

The deal marked a turning point in Syria’s civil war as President Bashar al-Assad, a Moscow ally, was poised to reassert his control over the country’s five largest cities after a six-year effort to unseat him.

In the last year, Turkey has seen a wave of terrorist attacks that the government has mainly blamed on Kurdish militants.

Turkish Foreign Minister Mevlüt Çavuşoğlu plans to hold talks about Syria in Moscow on Tuesday with his Russian and Iranian counterparts, Hurriyet reported.

A Turkish official on Monday denied that Ankara had reached any secret “bargain” with Moscow over the future of Syria, despite the improving cooperation that led to the deal for evacuations from Aleppo, AFP reported.

US State Department spokesman John Kirby said American officials were aware of reports about the shooting.

“We condemn this act of violence, whatever its source,” Kirby said. “Our thoughts and prayers are with him and his family.”

Karlov, who became ambassador in July 2013, started his career as a diplomat in 1976 and worked in North Korea for more than 30 years before moving to Ankara in 2007, according to a biography on the Russian Embassy’s website.