Russia will hold a funeral Thursday for Andrey Karlov, the country’s murdered ambassador to Turkey. Reports say the Turkish police officer who shot the ambassador provided security for Turkey’s president eight times in recent months.

Several people have reportedly been detained as part of the investigation into this murder, including the killer’s parents and other family members, reports CBS News correspondent Holly Williams. Turkey says the assassin didn’t act alone, but so far no group has claimed responsibility.

On Wednesday, Russia expressed its own doubts about the assassin working alone.

“We shouldn’t rush with any theories before the investigators establish who were behind the assassination of our ambassador,” spokesman Dmitry Peskov said.

Karlov’s body was sent home to Moscow Tuesday, a day after he was murdered in the Turkish capital. His widow was grief-stricken.

New video shows the 22-year-old Turkish police officer-turned-assassin watching Karlov as he gave a speech at an art exhibition. Seconds later he pulled out a gun and shot him in the back. The murderer was later killed by Turkish security forces.

The assassination was an apparent act of revenge for Russia’s bloody air strikes in Syria, which have helped the Syrian regime win back the city of Aleppo but forced tens of thousands of people to flee.

Many in Turkey and the rest of the Middle East are furious at Russia’s support for a government that has indiscriminately bombed its own people. Russia and Turkey are on opposite sides of the Syrian civil war. But recently relations between the two have improved.

In Moscow Tuesday night, the Russian and Turkish foreign ministers paid tribute to Karlov. At a summit meeting they also agreed on an expanded ceasefire in Syria.

Turkey says it believes a group known as Gulenists are behind the killing. Their leader, Fethullah Gülen, is an Islamic preacher who lives in Pennsylvania.

U.S. Secretary of State John Kerry talked with his Turkish counterpart Tuesday about allegations by some in Turkey that the U.S. was somehow involved, which a spokesman called ridiculous.

“It’s a ludicrous claim. Absolutely false,” State Department spokesman John Kirby said. “There’s no basis of truth in it whatsoever and the secretary made that very clear in his discussions today with the foreign minister.”

Whoever was behind this assassination, it’s highlighted just how close Russia has become to America’s NATO-ally Turkey. The U.S. was excluded from the summit meeting Tuesday.