Tensions rose sharply between Russia and the neighboring republic of Azerbaijan over the past week after two brothers, both ethnic Azerbaijanis, died in Russian custody, exposing a diplomatic crisis that threatens to further erode Moscow’s influence in the South Caucasus.

The two men, Huseyn and Ziyaddin Safarov, died after Russian investigators detained them last week as part of an inquiry into a series of cold case mafia-style assassinations in the industrial city of Yekaterinburg that occurred over the past 25 years.

One of the brothers died of heart failure, while the cause of death of the other one was being determined, Russian investigators said on Monday. On Wednesday, Russian investigators said they had charged six other detained Azerbaijanis, all Russian citizens, with murder.

Authorities in Baku, Azerbaijan’s capital, quickly denied the Russian version of events.

On Tuesday, the state prosecutor’s office of Azerbaijan said in a statement that the brothers had been subjected to “torture and murder with extreme cruelty” and that it had opened an investigation into the incident. And pro-government media outlets accused Moscow of deliberately targeting ethnic minorities as part of “chauvinist policies” used to “suppress internal dissent and strengthen totalitarian control.”

Azerbaijan’s sharp reaction laid bare Moscow’s shrinking sway over a country that only a few years ago was considered one of its closest partners among former Soviet states. In 2022, just two days before Russia’s invasion of Ukraine, Moscow and Baku signed a “declaration on allied interaction.”