Alexei Navalny’s spokesperson confirmed Saturday that the Russian opposition leader had died at a remote Arctic penal colony, saying he was “murdered,” but it was unclear where his body was as his family and friends searched for answers.

Navalny’s death at age 47 has deprived the Russian opposition of its most well-known and inspiring politician less than a month before an election that will give President Vladimir Putin another six years in power.

Although neither the imprisoned anti-corruption crusader nor other Kremlin critics were in position to challenge Putin for the presidency, the loss of Navalny was a crushing blow to Russians who had pinned their future hopes on Putin’s seemingly indefatigable foe. It also prompted questions about what killed him.

A note handed to Navalny’s mother stated that he died at 2:17 p.m. Friday, according to Navalny spokesperson Kira Yarmysh. Prison officials told his mother when she arrived at the penal colony Saturday that her son had perished from “sudden death syndrome,” Ivan Zhdanov, the director of Navalny’s Anti-Corruption Foundation, wrote on X, formerly known as Twitter.

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A prison colony employee said the body was taken to the nearby city of Salekhard as part of a post-mortem investigation, Yarmysh said. When Navalny’s mother and one of the late politician’s lawyers visited the morgue in Salekhard, it was closed, Navalny’s team wrote on its Telegram channel. But the lawyer called the morgue and was told the body was not there, his team said.

Another of Navalny’s lawyers went to Salekhard’s Investigative Committee and was told that the cause of Navalny’s death had not yet been established and that new investigations were being done with the results to be released next week, Yarmysh said. Russia’s Investigative Committee informed Navalny’s team that the body would not be handed over to his relatives until those investigations were complete, she said.

“It’s obvious that they are lying and doing everything they can to avoid handing over the body,” Yarmysh wrote on X, adding that his team demanded that Navalny’s body “be handed over to his family immediately.”

Russia’s Federal Penitentiary Service reported that Navalny felt sick after a walk Friday and fell 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.”

Maria Pevchikh, head of the board of Navalny’s Anti-Corruption Foundation, said the opposition leader would “live on forever in millions of hearts.”

“Navalny was murdered. We still don’t know how we’ll keep on living, but together, we’ll think of something,” she wrote on X.

Meanwhile, arrests continued Saturday as Russians came to lay flowers in memory of Navalny at memorials to the victims of Soviet-era purges. OVD-Info, a group that monitors political repression in Russia, said Saturday that more than 273 people had been detained at memorial events since Navalny’s death.

Memorial items laid Friday were removed overnight, but people continued trickling in with flowers on Saturday. In Moscow, a large group of people chanted “shame” as police dragged a screaming woman from the crowd, video shared on social media showed.

More than 10 people were detained at a memorial in St. Petersburg, including a priest who came to conduct a service for Navalny there.

In other cities across the country, police cordoned off some of the memorials and officers were taking pictures of those who came and writing down their personal data in a clear intimidation attempt.

“After the murder of Alexei Navalny, it’s absurd to perceive Putin as the supposedly legitimate head of the Russian state,” Ukrainian President Volodymyr Zelenskyy said at the Munich Security Conference in Germany Saturday. “He is a thug who maintains power through corruption and violence.”

U.K. Foreign Secretary David Cameron said Saturday that Britain “will be taking action” against the Russians responsible for Navalny’s death.

Speaking to broadcasters in Munich, Cameron said “there should be consequences” for “appalling human rights outrages like this.” He said Britain would “look at whether there are individual people that are responsible and whether there are individual measures and actions we can take.” Cameron did not say whether the response would consist of financial sanctions or other measures.

U.S. President Joe Biden said on Friday that Washington doesn’t know exactly what happened, “but there is no doubt that the death of Navalny was a consequence of something Putin and his thugs did.”

The Kremlin bristled Friday at the outpouring of anger from world leaders, with Putin’s spokesman, Dmitry Peskov, calling the statements — issued before medics have released the cause of Navalny’s death — “unacceptable” and “outrageous.”

Navalny had been jailed since January 2021, when he returned to Moscow after recuperating in Germany from nerve agent poisoning he blamed on the Kremlin. He was later convicted three times, saying each case was politically motivated, and received a sentence of 19 years for extremism.

After the last verdict, 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.”

Nigel Gould-Davies, a former British ambassador to Belarus and senior fellow for Russia & Eurasia at the International Institute for Strategic Studies in London, said the loss of Navalny shows that “the sentence in Russia now for opposition is not merely imprisonment, but death.”

Hours after Navalny’s death was reported, his wife, Yulia Navalnaya, made a dramatic appearance at the Munich conference.

She said she was unsure if she could believe the news from official Russian sources, “but if this is true, I want Putin and everyone around Putin, Putin’s friends, his government to know that they will bear responsibility for what they did to our country, to my family and to my husband.”

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Associated Press writer Katie Marie Davies in Manchester, England, contributed to this report.