One of the doctors at the local hospital that treated Russian opposition leader Alexei Navalny after he was poisoned with the Soviet-style Novichok nerve agent last year passed away this week, CNN reported.The report stated that the doctor, Sergey Maximishin, died "suddenly" and noted the hospital did not mention the cause of death. He was 55 years old."With regret, we inform you that ... the deputy chief physician for anesthesiology and resuscitation of the emergency hospital No.1, assistant of the department of Omsk State Medical University, PhD of medical sciences Maksimishin Sergey Valentinovich suddenly passed away," the Omsk hospital said in a statement, according to CNN. Navalny, a fierce critic of President Vladimir Putin and his lieutenants, began feeling ill on a domestic flight to Moscow in August after drinking tea at an airport café in the Siberian city of Tomsk.His condition became so serious that the plane made an emergency landing at the city of Omsk en route to Moscow, where he was carried off on a stretcher.He was placed in intensive care in a serious but stable condition, and on an artificial lung ventilator in a hospital in the city, about 2,200 km (1,370 miles) east of the Russian capital. Navalny was then moved to Berlin’s Charite hospital after being placed in a medically induced coma.According to CNN, Maximishin was confirmed by Navalny's chief of staff Leonid Volkov as being in charge of the Kremlin critic's treatment."Sergey Maximishin was the head of department that treated Alexey Navalny and was in charge of his treatment — specifically his medically induced coma," Volkov told CNN, adding that Maximishim "knew more than anyone else about Alexey's condition so I can't dismiss the possibility of foul play." "However Russia's health care system is very poor and it's not uncommon for doctors of his age to suddenly die," he continued, according to the report. "I doubt there will any investigation into his death."Navalny survived, and he was arrested at the Russian border on Jan. 17 upon his return from treatment in Germany. The anti-corruption campaigner accused Putin of ordering the attack. Moscow denies Navalny's charge, and has suggested that Navalny was a CIA asset, something he rejects.Navalny supporters protested against his arrest, taking to the streets across Russia in the biggest show of dissent against Putin since 2018. Thousands of demonstrators were detained.France, the United States, Britain, Germany and the European Union have called for Navalny's immediate release. US Secretary of State Antony Blinken said the new Biden administration would coordinate with close allies on holding Russia accountable.Despite the calls, a Russian court jailed Navalny on Tuesday in a ruling which the opposition politician blamed on what he called President Vladimir Putin's hatred and fear of him.The Moscow court handed Navalny a three-and-a-half-year sentence, but his lawyer said the anti-corruption blogger would serve only two years and eight months in jail because of time he has already spent under house arrest.Reuters and Zachary Keyser contributed to this report.