Russian authorities told the mother of late opposition figure Alexey Navalny he would be buried at the Arctic penal colony where he died unless she agreed within three hours to a secret funeral without a public farewell, his spokeswoman said Friday.

“An investigator called Alexey’s mother an hour ago and gave her an ultimatum. Either she agrees to a secret funeral without a public farewell within 3 hours, or Alexey will be buried in the colony,” Navalny’s spokeswoman Kira Yarmysh said Friday.

With Navalny’s wife and team exiled from Russia, his mother, Lyudmila Navalnaya, has been on a solitary mission to take her son’s body from Russian authorities and bury him with a funeral. But she said she has met stiff resistance since his death was announced a week ago, with Navalny’s team accusing the Kremlin of trying to prevent a funeral being held because they are “afraid” large crowds would attend.

Navalnaya had previously said that investigators had threatened to let her son’s body “decompose” unless she agreed to their demand that he be buried in secret.

“They started threatening me. Looking into my eyes they say that if I don’t agree to a secret funeral they will do something with my son’s body,” she said in a video posted Thursday on her son’s YouTube account.

Navalnaya said that officials in Salekhard, the Siberian town where Navalny’s body is being kept in a morgue, had shown her the body on Wednesday and that she had signed his medical death certificate, which claimed he had died of natural causes.

Yarmysh said that Navalnaya had refused to negotiate with the investigators and had demanded that her son’s body be returned to her on Saturday in “compliance with the law, according to which investigators are obliged to hand over the body within two days of establishing the cause of death.”

The news of the ultimatum given to Navalnaya sparked furious reaction from Navalny’s team.

“This is hell on earth right before our eyes,” Maria Pevchikh, the head of investigations at Navalny’s Anti-Corruption Foundation, said Friday, accusing Russian authorities of coming up with “new ideas for blackmail every day.”

Ivan Zhdanov, director of the foundation, announced it was offering a reward of 20,000 euros (about $21,000) for “valuable and complete information” about what he described as the “murder” of Navalny.

He called on members of the Russian military and law enforcement to “show solidarity with civil society” by anonymously contacting them with information about the circumstances that led to his death. Navalny’s team will also help those who come forward to leave the country – if they wish.

Russian authorities were accused by Navalny’s team of trying to prevent a funeral in order to prevent public displays of support.

“Many people [are] well within their right will want to say farewell to Alexey and look at him for the very last time,” Pevchikh said. She accused Russian President Vladimir Putin of “trying to prevent” Navalny’s funeral as “he sees it as a kind of a rally, a political turbulence he does not need now. He doesn’t want to see this on TV.”

Navalny, Putin’s most prominent political opponent, died aged 47 last Friday in a prison above the Arctic circle. The Russian prison service said he “felt unwell after a walk” and “almost immediately” lost consciousness.

He had been sentenced to 19 years in prison in August after being found guilty of creating an extremist community and other crimes, adding to the 11-and-a-half years to which he had previously been sentenced. Navalny had dismissed the charges against him as politically motivated.

Navalny’s death came just weeks before Russia’s presidential election scheduled for March 17, at which Putin is expected to sweep to a fifth term in office, extending his rule until at least 2030.

US President Biden on Friday announced a fresh slate of sanctions on more than 500 targets in response to Navalny’s death and Moscow’s ongoing invasion of Ukraine.

“These sanctions will target individuals connected to Navalny’s imprisonment as well as Russia’s financial sector, defense industrial base, procurement networks and sanctions evaders across multiple continents,” Biden said in a statement. “They will ensure Putin pays an even steeper price for his aggression abroad and repression at home.”

This story has been updated.