The doctor who treated Russian opposition leader Alexei Navalny immediately after his alleged poisoning last year has died, the hospital where he worked confirmed on Thursday.

Sergey Maximishin, the deputy chief physician of the Omsk emergency hospital in Russia, died “suddenly” at the age of 55, CNN quoted a statement from the hospital as saying. 

“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 statement from the hospital said. 

It did not confirm the cause of death. 

Navalny was admitted to the acute poisoning unit of the Omsk emergency hospital on August 20 after falling ill because of exposure to military-grade Novichok on plane bound for Moscow from Serbia. 

The plane had to make an emergency landing in Omsk. 

Maximishin, who was one of the top doctors at the hospital, did not hold press conferences at the time of Navalny’s poisoning.

Navalny was put in a medically-induced coma and transferred to Berlin, where he spent the next five months in recovery from the poisoning attempt. 

Following his return to Russia, Navalny was imprisoned for violating probation terms of a previous sentence, which has since sparked widespread criticism from around the world. 

Navalny blames the poisoning attempt on Russia’s secret services and President Vladimir Putin, charges that the Kremlin have repeatedly denied. 

Navalny’s chief of staff Leonid Volkov confirmed Maximishin was in charge of the leader’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. 

“(Maximishin) knew more than anyone else about Alexey’s condition so I can’t dismiss 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. I doubt there will be any investigation into his death,” he added. 

In a statement, the Minister of Health of the Omsk region said that Maximishin worked at the hospital for 28 years and saved thousands of lives. 

“He brought people back to full reality. We will miss Dr Maximishin very much. He left too early and because of this the pain of loss is especially bitter,” the statement from Alexander Murakhovsky said.