MOSCOW — Traces of the poison that put prominent Kremlin critic Alexei Navalny into a weeks-long coma were detected on a water bottle recovered from his hotel room in Siberia, according to a post Thursday on his Instagram page.

The new details could cast an even harsher light on the Russian government’s claim that there isn’t sufficient evidence to open a criminal case into Navalny’s poisoning, despite calls from Western governments to investigate. Navalny’s allies have said he was the victim of a state-ordered toxic attack, which the Kremlin has denied.

After Navalny became suddenly ill during an Aug. 20 flight to Moscow, his spokeswoman Kira Yarmysh initially said she suspected that a cup of tea he drank at an airport cafe in Tomsk that morning had been laced with poison because it was the only thing she saw him eat or drink.

But on Thursday morning, his camp revealed that once Navalny was hospitalized, his associates in Tomsk bagged up several items from his room at the Xander Hotel because they did not trust Russian investigators.

“It was decided to take everything that could be useful in some hypothetical way and pass it on to doctors in Germany,” Navalny’s Instagram post said. “It was also obvious that the case would not be investigated in Russia. And this is exactly what happened. Almost a month has passed, and Russia has not recognized the fact of Alexei’s poisoning.”

Germany later announced that Navalny had been poisoned with a nerve agent similar to Novichok, citing “unequivocal” evidence. On Monday, French and Swedish labs independently confirmed Germany’s findings.

A German government spokesman declined Thursday to comment on the Navalny camp’s revelation.

Novichok, a class of chemical weapons developed by the former Soviet Union and Russia, was used to poison former Russian double agent Sergei Skripal and his daughter in Salisbury, England, in 2018. Western intelligence also blamed Russian agents for that poisoning.

Navalny has been receiving treatment at Berlin’s Charité hospital since Aug. 22, and he made his first public statement Tuesday, writing on Instagram that he was able to breathe for a full day completely on his own after spending more than three weeks on a ventilator. Yarmysh confirmed that Navalny plans to return to Russia once he has recovered.

Navalny’s camp said the water bottle was one of the free ones the hotel leaves for guests in their rooms. Navalny took precautions when he traveled: The Russian newspaper Moskovsky Komsomolets, citing security agents surveilling Navalny during his August trip to Siberia, said he stayed in safe houses in Novosibirsk.

At the Xander Hotel, Navalny’s team took more rooms than they required, according to the newspaper, and Navalny did not stay in the room that was registered in his name.

The Organization for the Prohibition of Chemical Weapons said Thursday that Germany has asked for its assistance with the case and that it has taken samples for analysis. Germany’s Defense Ministry said last week that it had handed over Navalny’s test results to the Hague-based organization, of which Russia is a member.

Loveday Morris in Berlin contributed to this report.