A spokesperson for Russian dissident Alexei Navalny said on social media Monday that the activist has been missing within the prison system for six days. Spokesperson Kira Yarmysh said that Navalny’s team doesn’t “know where [he] is or what’s happening to him.”

A long-time critic of Russian President Vladimir Putin’s government, Navalny has been behind bars in Russia since 2021 when he returned to the country after being poisoned by a chemical nerve agent in Germany. He was sentenced to over 30 years for a variety of charges relating to “extremist” activities.

Navalny’s disappearance comes just a week after a new criminal case relating to alleged vandalism was launched against him under Article 214 of the Russian Penal Code, which he denies. He is currently imprisoned in the IK-6 jail in Melekhovo and was due to appear in court via video link but Yarmysh said in a tweet that prison authorities told Navalny’s lawyers that “Navalny is no longer on their list. They refuse to say where he was transferred.”

Putin announced on Friday that he would be running for president in Russia’s 2024 elections, and Navalny’s Chief of Staff, Leonid Volkov, said that the timing of Navalny’s disappearance was not a coincidence, especially considering Navalny was due to be transferred two months ago. In a social media post he stated that the disappearance was:

0% accidental and 100% directly political manual control from the Kremlin. It is no secret to Putin who his main opponent is in these “elections.” And he wants to make sure that Navalny’s voice is not heard. This means that everyone should become Navalny’s voice.

Back in October, three Navalny lawyers were arrested for participating in an “extremist” group led by the activist.