Updated

The Russian opposition critic suspected to have been poisoned by the Kremlin does in fact have poison in his system, a German hospital confirmed Monday.

The Charité hospital in Berlin, where a team of doctors has been examining Alexei Navalny since he was flown from Siberia and admitted Saturday, announced that tests have found the presence of “cholinesterase inhibitors” in his system.

“The patient is in an intensive care unit and is still in an induced coma,” the hospital added. “His health is serious but there is currently no acute danger to his life.”

An empty stretcher is carried back into an ambulance that is believed to have transported Alexei Navalny at the emergency entrance of the Charite hospital in Berlin, Germany, on Saturday. (AP)

An empty stretcher is carried back into an ambulance that is believed to have transported Alexei Navalny at the emergency entrance of the Charite hospital in Berlin, Germany, on Saturday. (AP)

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Cholinesterase inhibitors, according to the Associated Press, are a broad range of substances that are found in several drugs, but also pesticides and nerve agents. However, doctors at Charite say the specific substance to which Navalny was exposed is not yet known.

The medical saga surrounding Navalny began Thursday when he fell ill on a flight to Moscow.

Allies of Navalny, who has promoted opposition candidates to Russian President Vladimir Putin since being kept from running against the longtime leader in a 2018 presidential election, suspect the tea he was drinking before he boarded the flight was poisoned by someone linked to Putin.

Navalny was first approved to be transferred to the Berlin hospital Friday, but when a German plane equipped with medical supplies arrived, his Russian doctors said he was too fragile to travel, leading supporters to believe his transfer was delayed for political reasons – and until any poison would be untraceable in his system.

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Navalny’s team last week submitted a request in Russia to launch a criminal probe, but as of Monday, Russia’s Investigative Committee still has not opened a case, Navalny's spokeswoman Kira Yarmysh said.

Ilya Yashin, an opposition politician in Moscow and a close ally of Navalny, in a video statement Monday urged Russia’s law enforcement to investigate “an attempt at a life of a public figure” and to look into the possible involvement of Putin.

“It is Putin who benefits from these endless assaults,” Yashin said.

U.S. Ambassador to Russia John Sullivan said the Navalny case would on the agenda for Deputy Secretary of State Stephen Biegun’s visit to Russia that begins Tuesday.

Alexei Navalny was flown to Germany Saturday following a dispute over whether he was stable enough to board an aircraft. ( Michael Kappeler/dpa via AP)

Alexei Navalny was flown to Germany Saturday following a dispute over whether he was stable enough to board an aircraft. ( Michael Kappeler/dpa via AP)

“With Alexei Navalny in a hospital in Berlin, our dialogue with Russia must include reemphasizing the importance of free speech and civil society,” he told reporters.

If Navalny is found to have been poisoned “that would represent a crucial moment in Russia," he said.

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"The Russian people deserve to see that anybody who would have been involved in a matter like that be held accountable.”

The developments come after Russian doctors on Monday claimed two laboratories found no poisonous substances in Navalny’s system.

“If we had found poisoning confirmed by something, it would have been much easier for us,” said Anatoly Kalinichenko, deputy chief doctor of the Omsk Ambulance Hospital No. 1, where Navalny was first taken after the plane he was on made an emergency landing. “But we received a final conclusion from two laboratories that no toxic chemicals that can be considered poisons or by-products of poisons, were found.”

The chief doctor at that facility also claimed Monday there was no outside interference in Navalny’s treatment there.

“We were treating the patient, and we saved him," Alexander Murkhavsky said Monday while rejecting allegations made by Navalny’s team that doctors in Omsk had been acting in coordination with Russia’s security services. “There wasn’t and couldn’t be any influence on the patient’s treatment.”

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But Murkhavsky wasn’t able to identify men in plainclothes spotted in the hospital last week who the politician’s allies said were law enforcement and security service agents.

“I can’t say who they were,” Murakhovsky said, according to the Associated Press.

Meanwhile, police are posted outside the hospital in Germany where Navalny is currently being treated.

“The circumstances of what led to Alexei Navalny's critical condition haven't yet been clarified,” Dirk Wiese, the German government's coordinator for Eastern European affairs, told public broadcaster ZDF earlier today.

Like many other opposition politicians in Russia, Navalny has been frequently detained by law enforcement and harassed by pro-Kremlin groups. In 2017, he was attacked by several men who threw antiseptic in his face, damaging an eye.

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Last year, Navalny was rushed to a hospital from jail where he was serving a sentence on charges of violating protest regulations. His team also suspected poisoning then. Doctors said he had a severe allergic reaction and sent him back to detention the following day.

Fox News’ Brie Stimson and the Associated Press contributed to this report.