The widow of Russian opposition leader Alexei Navalny vowed on Monday to continue his fight against the Kremlin while authorities denied his mother access to a morgue where his body is believed to be held after his death last week at an Arctic penal colony.

Fighting back tears, Yulia Navalnaya accused Russian President Vladimir Putin of killing her husband in the remote prison and vowed to punish him and other alleged perpetrators.

She also slammed the authorities, saying they were refusing to hand over the body to Navalny’s mother to cover-up his alleged killing, and referred to his alleged earlier poisoning with a Soviet-era Novichok nerve agent.

Russian authorities said that the cause for Navalny’s death Friday at age 47 is still unknown. He had been jailed since January 2021, when he returned to Moscow after recuperating in Germany from poisoning with a Novichok-type nerve agent that he blamed on the Kremlin. He received three prison terms since his arrest, on a number of charges he has rejected as politically motivated.

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“They are cowardly and meanly hiding his body, refusing to give it to his mother and lying miserably while waiting for the trace of another Putin’s Novichok to disappear,” Navalnaya said.

She urged Russians to rally behind her “to share not only the grief and endless pain that has enveloped and gripped us, but also my rage.”

“Rage, anger, hatred for those who dared to kill our future,” she said. “I address you with the words of Alexey, in which I really believe: It’s not a shame to do little, it’s a shame to do nothing. It’s a shame to let yourself be intimidated.”

Navalnaya urged all those who mourn Navalny to unite to fulfill his dream of a “beautiful Russia of the future” so that “the unimaginable sacrifice” he made would not have been in vain.

“The main thing that we can do for Alexei and ourselves is to keep fighting,” she said. “Stronger, more fiercely and valiantly that we did before. We all need to get together in one strong fist and strike that mad regime, Putin, his cronies, bandits in epaulets, thieves and killers who mutilated our country.”

Navalny’s spokesperson Kira Yarmysh said that the Investigative Committee, the country’s top criminal investigation agency, informed Lyudmila Navalnaya that the cause of her son’s death remained unknown and that the official probe had been extended. “They lie, buy time for themselves and do not even hide it,” Yarmysh posted on X, formerly Twitter.

Many world leaders blamed President Vladimir Putin and his government for Navalny’s death. On Monday, EU foreign policy chief Josep Borrell said the bloc was mulling sanctions against Russia.

He noted the responsibility for Navalny’s death lies with “Putin himself, but we can go down to the institutional structure of the penitentiary system in Russia,” to track down those involved and impose asset freezes and travel bans.

Kremlin spokesman Dmitry Peskov slammed what he described as “boorish” and “inadmissible” statements by Western leaders who held Putin responsible for Navalny’s death.

“Those statements can’t do any harm to the head of our state, but they certainly aren’t becoming for those who make them,” Peskov said in a call with reporters.

Yarmysh said that Navalny’s 69-year-old mother and his lawyers were not allowed into the morgue in Salekhard on Monday morning. The staff didn’t answer when they asked if the body was there, Yarmysh said.

Asked when Navalny’s body could be handed over to his family, Peskov responded that the Kremlin was not involved in those proceedings, adding that the official probe was continuing in line with the law.

Navalny’s ally Ivan Zhdanov denounced the Russian authorities as “lackeys and liars.” “It’s clear what they are doing now — covering up the traces of their crime,” he wrote Monday.

Navalny’s death has deprived the Russian opposition of its most well-known and inspiring politician less than a month before an election that is all but certain to give Putin another six years in power. It dealt a devastating blow to many Russians, who had seen Navalny as a hope for political change following his unrelenting criticism of the Kremlin.

Nearly 300 people have been detained by police in Russia as they streamed to ad-hoc memorials and monuments to victims of political repression with flowers and candles to pay tribute to Navalny, according to OVD-Info, a group that monitors political arrests. The U.S. and British ambassadors also mourned Navalny’s death at a memorial in Moscow.

Authorities cordoned off some of the memorials across the country and were removing flowers at night, but they kept appearing.

Over 50,000 people have submitted requests to the Russian government asking for Navalny’s remains to be handed over to his relatives, OVD-Info said.

Russia’s Federal Penitentiary Service reported that Navalny felt sick after a walk Friday and became unconscious at the penal colony in the town of Kharp, in the Yamalo-Nenets region about 1,900 kilometers (1,200 miles) northeast of Moscow. An ambulance arrived, but he couldn’t be revived, the service said, adding that the cause of death is still “being established.”

Some Russian media claimed that Navalny’s body bore bruises, possibly caused by medics’ attempts to resuscitate him. The reports couldn’t be independently confirmed.

After the last verdict that handed him a 19-year term, Navalny said he understood he was “serving a life sentence, which is measured by the length of my life or the length of life of this regime.”

In Brussels on Monday, Navalny’s widow met with European Union foreign ministers and other EU officials who were considering sanctions against Moscow over Navalny’s death.

“By killing Alexei, Putin killed half of me, half of my heart and half of my soul,” Navalnaya said in a video statement. “But I still have the other half, and it tells me that I have no right to give up. I will continue the work of Alexei Navalny.”