A Russian online media outlet has reported that opposition leader Alexei Navalny was killed in prison by being poisoned, contradicting the official story that he died of natural causes.

The Insider, a site focused on investigative journalism, said in a report on Sunday that it obtained official documents related to his death in February at a prison in the Russian Far North.

The report said an early version of the documents described Navalny's health as sharply deteriorating, and that prison medical staff were informed of his complaints about sharp pain in his abdomen, vomiting and convulsions.

But The Insider reports that mentions of his symptoms had been removed in later documents.

The Insider also quoted a Russian doctor who treated Navalny after an attempted poisoning in 2020 as saying that sharp abdominal pain, vomiting or seizures "can hardly be explained by anything other than poisoning."

The doctor reportedly added that "the short interval between the abdominal pain and the convulsions suggests possible exposure to an organophosphorus agent, for instance -- the same class of substances as Novichok."

Navalny's widow, Yulia Navalnaya, accused Russian President Vladimir Putin of involvement in the case shortly after Navalny's death.

The head of Ukraine's defense intelligence said Navalny died from a blood clot, refuting speculation that he was killed.