Russian investigators arrived in Turkey Tuesday morning after the assassination of Moscow’s ambassador in Ankara. Andrey Karlov was shot from behind at the opening of an art exhibit on Monday. The U.S. calls the murder a “heinous attack on a member of the diplomatic corps.”

Russia and Turkey say this assassination was an attempt to disrupt their relationship -- but on the face of it, it looks like an act of retribution for Russia’s bloody air campaign in Syria, reports CBS News correspondent Holly Williams.

Karlov was giving a speech at an art gallery, watched by the man who would kill him minutes later.

“Remember Aleppo! Remember Syria!” the assassin shouted after shooting Karlov in the back, murdering him, apparently in revenge for Russia’s deadly intervention in the Syrian civil war.

The gunman was a 22-year-old Turkish police officer whose attack left others in the museum cowering. He was later killed in a shoot-out with security forces, according to local media.

Russia and Turkey are on opposite sides in the Syrian civil war but recently relations have improved. The two countries negotiated a ceasefire last week to help evacuate Syrian civilians.

But this comes just days after thousands in Turkey protested against Russian airstrikes in Syria. Like many in the Middle East, they’re furious at Russia’s lethal support for the Syrian regime.

It’s helped the regime claw back control of the city of Aleppo, but Syrians have paid with their lives and it’s forced thousands of civilians to flee. The U.S. has condemned it as barbarous.

In northern Syria Monday, we visited Al Kamuna camp where around 200 people who fled Aleppo Province live in mud and desperation. But even Al Kamuna – a place of last resort – was hit by an airstrike in May, people there told us. It’s the suspected work of either Russia or the regime and further fueling anger with Moscow.

Shots were also fired outside the U.S. embassy in the Turkish capital Monday night. A gunman reportedly approached the main gate and produced a shotgun before firing several times into the air. There were no casualties and the man was detained. The embassy will be closed today.