Adding to the anguish felt by the late Russian opposition leader Alexey Navalny's family, his mother and his team have reportedly been denied access to his body and told by investigators that the official probe into his death is being extended, and it's unclear how long it will take.

"They are cowardly and meanly hiding his body, refusing to give it to his mother," Yulia Navalnaya, the widow of the fierce Kremlin critic said in a video statement four days after Russian prison authorities announced his death in prison — which they attributed to "sudden death syndrome."

• Alexey Navalny's message to the world, "if they decide to kill me"

Navalnaya, who lives in exile outside Russia, accused the Russian authorities of "lying miserably while waiting for the trace of another Putin's Novichok to disappear," referring to a poison allegedly used by Russian security services in at least one previous politically motivated assassination attempt.

Navalnaya urged Russians "to share not only the grief and endless pain that has enveloped and gripped us — but also my rage," as she vowed to continue with her husband's mission to reveal Putin's alleged misdeeds and end his long reign of power over Russia.

A spokeswoman for Navalny, in a social media post, also accused that Russian officials of lying and "playing for time," as authorities continued to crack down on tributes to the late dissident.

At Moscow's Solovetsky Stone — a monument to victims of political repression — people laid flowers in memory of Navalny. 

But in Putin's Russia, that repression is everywhere, and the tributes, along with dozens of others left across the nation, were quickly swept away. Hundreds of people who have dared to honor Navalny publicly since he died on Friday have been arrested.

"I think it illustrates a deep mourning among the people who were supporters of Navalny, which was a large section of Russians," Russia analyst Jeff Hawn told CBS News, adding that many of those people "believe they've now lost hope, because, in many ways, Navalny was able to bring together a broad coalition of people who wanted a Russia that was a normal, a more normal country."

 Navalny was last seen alive just one day before his death, appearing from prison via remote video link for a court appearance. 

He looked gaunt but seemingly healthy and in good spirits at the IK-3 "Polar Wolf" penal colony in Russia's far north, where he was being held after a handful of convictions — all of which he, and his many supporters around the world, always dismissed as groundless and politically motivated.

Prison officials said he went for a walk Friday, felt suddenly ill and collapsed, and then could not be revived by prison medics. They later attributed it to "sudden death syndrome."

Navalny's allies, President Biden and many other world leaders, however, say Putin bears responsibility for his most prominent critic's demise.

The Russian president was pictured smiling during a visit with factory workers shortly after the news of Navalny's death broke on Friday, but he still hasn't commented publicly.

After exposing corruption at almost every level of the Russian state, frequently targeting Putin himself, Navalny survived at least two poisoning attempts and spent years in some of Russia's most notorious prisons before dying at 47, leaving behind his wife Yulia and two children.

World leaders, including Mr. Biden, have vowed to hold those responsible for Navalny's death to account, but with Russians due to go to the polls in just a few weeks and Putin all but certain to secure yet another full term, accountability at any level appeared a long way off on Monday.

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• Alexei Navalny
• Alexey Navalny
• Russia
• Vladimir Putin