Supporters of prominent Russian opposition figure Vladimir Kara-Murza Jr., who is serving a 25-year sentence for treason, said Monday that he has disappeared from the Siberian prison where he was behind bars.

Kara-Murza, 42, was held in a prison in the Omsk region, but a letter sent to him by activist and journalist Alexander Podrabinek was returned with the notation that Kara-Murza was no longer there, Podrabinek said on Facebook.

A lawyer for Kara-Murza, Vadim Prokhorov, said another lawyer who tried to visit him on Monday was told that he wasn’t in the prison, according to the Telegram news channel Agentstvo.

Transfers within Russia’s prison system are shrouded in secrecy and inmates can disappear from contact for several weeks. Backers of Russia’s most noted opposition figure, Alexei Navalny, were alarmed in December when he couldn’t be found.

Navalny, serving a 19-year sentence, resurfaced in a prison colony above the Arctic Circle. He previously had been held in the Vladimir region in central Russia about 230 kilometers (140 miles) from Moscow.

Kara-Murza was arrested in 2022 and later sentenced to 25 years on charges stemming from a speech that year to the Arizona House of Representatives in which he denounced Russia’s full-scale invasion of Ukraine.

The political activist and journalist, who twice survived poisonings he blamed on Russian authorities, has rejected the charges against him as punishment for standing up to President Vladimir Putin and likened the proceedings to the show trials under Soviet dictator Josef Stalin.

Under Putin, moves to neutralize opposition and stifle criticism intensified after the start of the war in Ukraine, including passage of a law criminalizing reports seen as defaming the Russian military.