BERLIN — German authorities said Wednesday they suspect that Russian agents were behind an execution-style killing in Berlin in the summer and expelled two Russian diplomats in connection with the case.

The federal public prosecutor said there was “sufficient factual evidence” that the Aug. 23 killing was carried out by Russian intelligence agencies or those of Russia’s Chechen Republic.

The German Foreign Ministry declared two employees of the Russian Embassy in Berlin “persona non gratae,” saying that Russian authorities have not cooperated with the investigation “sufficiently” despite repeated “high-ranking” requests.

 Russian presidential spokesman Dmitry Peskov rejected the accusation of Russian involvement in the killing, describing it as “absolutely groundless.” The Russian Foreign Ministry said it would be forced to take “tit-for-tat steps” in retaliation.

The killing compounds concerns about Russia’s efforts to assassinate political opponents on European soil less than two years after Britain accused Moscow of attacking former Russian agent turned informant Sergei Skripal with nerve agent in Salisbury, England. London also concluded that Russia was behind the 2006 killing of another former Russian agent, Alexander Litvinenko, who was poisoned with radioactive material.

The Berlin killing — if Russian involvement is proved — would represent the first such slaying on German soil since the end of the Cold War. It threatens to stymie relations between Berlin and Moscow, a fragile bond for Germany, which tries to balance political differences against its energy needs.

Berlin prosecutors named the victim, who was shot in the head in a central Berlin park, as “Tornike K.” — a 40-year-old Russian-Georgian citizen who was designated a “terrorist” by Moscow for his role in armed conflict with Russia. He also used the alias Zelimkhan Khangoshvili. Between 2000 and 2004, he commanded a Chechen militia that fought Russian forces during the Russian-Chechen war, the German prosecutor said in a statement.

The slain man also fought with a Georgian unit defending South Ossetia during the 2008 Georgia-Russia war, the statement said.

He survived a 2015 assassination attempt, despite being shot four times, and later fled to Germany, where he claimed asylum.

The Interfax news agency cited an unnamed Russian law enforcement official stating that Khangoshvili participated in attacks in Ingushetia, a Russian republic neighboring Chechnya, in 2004. The attacks killed 98 people, including 67 law enforcement officers.

The official said Khangoshvili was following the orders of Chechen rebel leader Shamil Basayev, whom he described as an international terrorist, Interfax reported. Basayev, who masterminded some of the deadliest attacks against Russia, including many that targeted civilians, was killed by Russian special forces in a truck explosion in Ingushetia in 2006.

According to German press reports, he was tailed by a man on an electric bicycle before being shot with a Glock-26 pistol. German authorities arrested a man identified as Vadim Sokolov on suspicion of murder shortly afterward. He was traveling on a Russian passport that authorities said they thought was genuine, but they suspected that his identity was not.

The investigative website Bellingcat reported Tuesday that Sokolov’s real name is Vadim Krasikov. The German prosecutor also said it was “highly likely that they are one and the same person,” based on photographic comparisons.

Krasikov was the subject of an international arrest warrant issued by Russian authorities in 2014 for a slaying one year prior in Moscow — a killing that bore hallmarks similar to the Berlin assassination. The killer also approached the victim on a bicycle. Russian authorities later amended and then deleted the wanted notice.

“The findings that the Russian state terminated domestic and international search warrants for Vadim Krasikov, and then facilitated the issuance of a new, false identity to the same person, provides further convincing evidence that the Russian state was involved in the planning and facilitation, if not gave the mandate for the assassination of a foreign citizen on German territory,” Bellingcat said.

Sergey Nechayev, the Russian ambassador in Berlin, said Moscow was “deeply disappointed” by the approach of the Germans.

“There was no evidence of Russian government involvement in this incident,” he said.

Russian Foreign Minister Sergei Lavrov said Moscow would take “a little time” to decide its countermeasures in response to the expulsions. “We are thoughtful people and will first study what we have been accused of and why in the world all this happened,” he said.

Speaking before Germany announced the expulsion of the diplomats, Peskov said he did not think the issue would cast a pall over a meeting next week between Russian President Vladi­mir Putin and German Chancellor Angela Merkel.

Last year, Germany expelled four diplomats after the poisoning of Skripal and his daughter. It was part of a wave of coordinated European and NATO expulsions after British authorities concluded that Moscow was responsible. Skripal and his daughter survived the attack, which was carried out with a Russian-developed nerve agent. Russia denied involvement.

The diplomatic fallout with Russia from the Berlin assassination could be particularly sensitive for Germany, which is attempting to increase imports of cheap Russian natural gas to meet targets of weaning itself off coal. The United States has been vociferously opposed to Nord Stream 2, a controversial new gas pipeline between Russia and Germany, as it would deepen European dependence on Russian energy.

Dixon reported from Moscow. Luisa Beck in Berlin contributed to this report.