Germany expelled two Russian diplomats on Wednesday after prosecutors said Moscow could be behind the killing of a former Chechen rebel commander in a Berlin park.
Russia's foreign ministry immediately pledged unspecified "retaliatory measures", saying the accusations were "groundless and hostile".
Zelimkhan Khangoshvili, a 40-year-old Georgian national, was shot twice in the head at close range in Kleiner Tiergarten park on August 23, allegedly by a Russian man who was arrested shortly afterwards.
The suspect in the killing was said to be riding a bicycle and was seen by witnesses afterwards throwing the bike and a stone-laden bag with a gun into a river.
Police also recovered a wig he was alleged to have used.
He has until now been named by police only as Vadim S. but evidence revealed by German prosecutors on Wednesday indicated this may have been a fake identity.
The case has been compared with the poisoning of former Russian agent Sergei Skripal in Britain last year with a Soviet-era nerve agent, which plunged relations between London and Moscow into a deep freeze.
The attempted murder led to dozens of tit-for-tat diplomatic expulsions reminiscent of the Cold War.
Speaking after a NATO summit, Chancellor Angela Merkel said Germany had acted "because we have not seen Russia supporting us in clearing up this murder".
She said she did not expect any effect on a Ukraine summit planned for December 9 in Paris in which Russian President Vladimir Putin is also due to take part.
- 'Sufficient factual evidence' -
In the case of the Berlin murder, German media and some politicians had already raised suspicions of Russian state involvement but said the government was dragging its heels because of strong economic ties with Russia.
But a statement from the German foreign ministry on Wednesday indicated a change of tone.
"The foreign ministry has today declared two employees of the Russian embassy in Berlin as personae non gratae with immediate effect," it said.
"Despite repeated high-ranking and persistent demands, Russian authorities have not cooperated sufficiently in the investigation into the murder."
The investigation is now in the hands of German federal prosecutors, who handle intelligence cases.
"There is sufficient factual evidence 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," the prosecutor's office said in a statement.
Chechnya has been led with an iron fist since 2007 by Ramzan Kadyrov, a close Putin ally.
- Russian ministry link -
Outlining the results of their investigation so far, prosecutors said Vadim had travelled from Moscow to Paris on August 17 and then on to Warsaw on August 20 before travelling to Berlin.
Prosecutors said his visa for travelling to Europe indicated he was a civil engineer working for a company in Saint Petersburg.
But the company was not operational and a fax number for the firm was registered to another company belonging to Russia's defence ministry.
Prosecutors said the man's features matched those of a suspect in a 2013 murder in Moscow in which the suspect also approached the victim on a bicycle.
The investigative website Bellingcat on Tuesday said the suspect in both murders was 54-year-old Vadim Krasikov, who grew up in Kazakhstan when it was part of the Soviet Union before moving to Siberia.
German media said the suspicion was that Russian intelligence agencies had arrested him and recruited him after the 2013 killing.
- Victim fought Russian troops -
Bellingcat said the victim had fought in the second Chechen war in 1999-2002, then continued supporting Chechen separatists from his native Georgia.
He too lived for a time under an assumed identity as Tornike Kavtarashvili, according to media reports.
Bellingcat said he "recruited and armed" a volunteer unit to fight Russian troops in Georgia in 2008.
After surviving two assassination attempts in Georgia, he moved to Germany and applied for asylum.
Khangoshvili is not the first Chechen exile to be killed in suspicious circumstances.
Former Chechen separatist president Zelimkhan Yandarbiyev was killed by a car bomb in Qatar in 2004 and Sulim Yamadayev was shot dead in Dubai in 2009.
Also in 2009, Kadyrov's former bodyguard Umar Israilov was shot and killed outside a market in Vienna.
Israilov had accused Kadyrov of involvement in torture and other human rights abuses.