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Months after a man was shot dead in a Berlin park, Germany is throwing out two Russian diplomats because it suspects the murder was ordered by Russia or Russia's Chechen republic.

The expulsions come shortly after Germany's chief prosecutor decided to take over the case.

Germany has accused Russia of failing to assist in the investigation.

A Russian official condemned the move as "Russophobic hysteria" and did not rule out a reciprocal move.

Zelimkhan Khangoshvili, a 40-year-old former Chechen rebel commander, was shot in the head from behind in the Kleiner Tiergarten park in August.

A man was quickly arrested but has given little information to police.

In its statement on Wednesday (in German), the German federal prosecutor identified the victim as "Tornike K". Khangoshvili is said to have spent several years living in Georgia under the name of Tornike Kavtarashvili.

"There are sufficient factual grounds to suggest that the killing... was carried out either on behalf of state agencies of the Russian Federation or those of the Autonomous Chechen Republic, as part of the Russian Federation."

Why is Russia suspected?

The suspect was detained shortly after the killing when he was allegedly seen dumping a bike, pistol and a wig into the nearby River Spree.

The prosecutor says the suspect flew from Moscow to Charles de Gaulle airport in Paris six days before the killing under the alias of Vadim S.

Vadim S appeared for the first time in a Russian domestic passport in 2015. Reports on Tuesday suggested his full name was Vadim Sokolov, 49, although that was not thought to be his real identity.

Vadim S is then said to have flown to Warsaw where he stayed in a hotel until 22 August.

After the murder in August a Russian spokesman said it had nothing to do with Russia or its authorities.

Leonid Slutsky, the head of the Russian Duma (parliament) foreign affairs committee told Interfax news agency that the expulsion of two diplomats was an unfriendly step.

The diplomatic row also comes five days ahead of a major Ukraine summit involving the leaders of Germany, Russia, France and Ukraine.

Who is the suspect?

The federal prosecutor referred in its statement only to Vadim K and his alias Vadim S. According to Bellingcat, the real identity of the man in custody is Vadim Krasikov, born in August 1965 in Soviet Kazakhstan.

The prosecutor describes Vadim K as a suspect in the 2013 murder of a Russian businessman in Moscow. The victim in that attack was also approached by a man on a bicycle who shot him in the back of the head.

An arrest warrant was issued by Moscow in 2014 but withdrawn in July 2015, the prosecutor says.

German media have compared the attack on Khangoshvili to the attempted murder of Russian former intelligence agent Sergei Skripal and his daughter in the UK last year.

Who was the victim?

A Georgian national, Khangoshvili fought in the second Chechen war against Russian forces in the North Caucasus in the period 2001-2005.

He was a close ally of Chechen President Aslan Maskhadov, who masterminded Chechen guerrilla resistance to Russia and was killed in a Russian special forces raid.

Khangoshvili fled to Germany after surviving an assassination attempt in the Georgian capital Tbilisi in 2015. He applied for asylum in Germany but was not granted it.

Chechnya is now run by Ramzan Kadyrov, a former rebel who switched sides and is now fiercely loyal to Russian President Vladimir Putin.