OMSK, Russia (NewsNation Now) — Dr. Sergey Maksimishin, a top doctor at the Omsk hospital where Russian opposition leader Alexei Navalny was treated after his recent poisoning, died “suddenly,” the hospital said.

Omsk Emergency Hospital No. 1 reported on its website Thursday that the deputy chief physician for anesthesiology and resuscitation died. He was 55, according to national security analyst JJ Green.

“We are saddened to report that in the 56th year the deputy chief physician for anesthesiology and resuscitation BSMP-1, assistant of the Department of OmGMU, Ph.D. Maximishin Sergey Valentinovich died suddenly,” the hospital said in a statement.

The hospital statement didn’t release a cause of death, but some media reports indicate he died of a heart attack. NewsNation is working to confirm how Maksimishin died.

According to the hospital, Maksimishin made great contributions to the anesthesiology and resuscitation service, including a number of methods to treat patients with acute cerebral disorders in critical condition.

The hospital treated Navalny, an anti-corruption investigator and Russian President Vladimir Putin’s most prominent critic, as he recovered from a nerve-agent poisoning that he blames on the Kremlin. The 44-year-old was arrested last month upon returning from a five-month convalescence in Germany.

Navalny’s Jan. 17 arrest triggered massive protests across Russia, with tens of thousands of people rallying in the streets to demand his release. Many protesters chanted slogans against Putin in the largest show of discontent in years. Thousands of protesters have been detained.

A Moscow court on Tuesday found that Navalny violated probation terms of his suspended sentence from a 2014 money-laundering conviction, and ordered him to serve two years and eight months in prison.

The Associated Press contributed to this report.