Allies of the imprisoned Russian opposition leader Alexei Navalny said they were searching for him for a 13th day after he failed to appear in court Monday.

Navalny’s spokesperson Kira Yarmysh wrote on X, formerly known as Twitter, that Navalny had multiple hearings scheduled, some of which were suspended after the politician who is President Vladimir Putin’s fiercest foe could not be located to participate in person or by video link.

The whereabouts of Navalny, 47, have been unknown since his lawyers lost touch with him after Dec. 6. They believe he is deliberately being hidden after Putin announced his candidacy in Russia’s March presidential election, which the longtime leader is almost certain to win.

“Alexei is Putin’s main opponent even though his name won’t be on the ballot,” Yarmysh told The Associated Press. “They will do everything they can to isolate him.”

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Navalny’s team has launched a campaign to encourage Russians to boycott the election or vote for another candidate.

Allies said a defense lawyer was told in court on Dec. 15 that Navalny had been moved from the penal colony east of Moscow where he was serving a 19-year term on charges of extremism, but the lawyer was not told where Navalny was taken.

Yarmysh told the AP that Navalny’s team had written to more than 200 pretrial detention centers and special prison colonies as well as checked all detention centers in Moscow in person in order to find the opposition leader.

Although a judge suspended Monday’s court proceedings for an indefinite period after Navalny could not be located, that does not mean judicial officials will find him, Yarmysh said.

“The court simply relieved itself of responsibility for administering justice,” she said.

Navalny’s allies sounded the alarm after his lawyers were not let into Penal Colony No. 6, the prison about 230 kilometers (140 miles) east of Moscow where he was serving his sentence, after Dec. 6. The lawyers also said that letters to him were not being delivered there and that Navalny was not appearing at scheduled court hearings via video link.

Yarmysh said earlier this month that those developments caused concern because Navalny had recently fallen ill and apparently fainted “out of hunger.” She said he was being “deprived of food, kept in a cell without ventilation and has been offered minimal outdoor time.”

He was due to be transferred to a “special security” penal colony, a facility with the highest security level in the Russian penitentiary system.

Russian prison transfers are notorious for taking a long time, sometimes weeks, during which there’s no access to prisoners, with information about their whereabouts limited or unavailable. Navalny could be transferred to any of a number of such penal colonies across Russia.

Navalny has been behind bars in Russia since January 2021, when he returned to Moscow after recuperating in Germany from nerve agent poisoning that he blamed on the Kremlin. Before his arrest, he campaigned against official corruption and organized major anti-Kremlin protests.

He has since received three prison terms and spent months in isolation in Penal Colony No. 6 for alleged minor infractions. He has rejected all charges against him as politically motivated.