According to Eva Merkacheva, a Russian journalist who has covered the country’s prisons extensively, inmates in such cells are often left cold and hungry, and those conditions could account for reports of Mr. Navalny’s death.

”I think his endless transfers to a punishment cell could definitely lead to this,” Ms. Merkacheva told MSK1, a news site.

Though incarcerated, Mr. Navalny had continued to post messages on social media by passing notes to his visiting lawyers, and he had described the brutal conditions in punishment cells.

Prison authorities used trivial transgressions as reasons to punish Mr. Navalny, he wrote in one Instagram post. He had been punished, he said, for wearing an unbuttoned robe, for not walking with his hands behind his back while being transferred between cells, and for failing to introduce himself properly.

Apart from spending time in a frigid, cramped cell, inmates in punishment cells are also limited in their ability to exercise, spending time in a tiny walled courtyard with a roof of prison bars. In his new penal colony in the Arctic, for instance, Mr. Navalny was allowed to go out only in the mornings, while it was still dark and the temperatures were at their lowest point.

“It’s never been colder than -25°F.,” Mr. Navalny wrote on social media in January, describing his walks. “Even at that temperature you can walk for more than half an hour, but only if you have time to grow a new nose, ears and fingers.”

In his posts, Mr. Navalny called his confinement to punishment cells a form of torture, though he also joked cavalierly that it was an opportunity for him to meditate.

But harsh prison conditions did damage Mr. Navalny’s health. He was first sent to a penal colony in March 2021, just months after being poisoned by a nerve agent that nearly killed him. Following the poisoning, Mr. Navalny lost significant weight and had to relearn basic movements, such as how to use his fingers with his phone.

During his first weeks in the penal colony, Mr. Navalny’s health quickly deteriorated. Leonid Volkov, his chief of staff, said that weeks after he was transferred to his first penal colony, Mr. Navalny suffered from acute back and leg pain.

While imprisoned, Mr. Navalny said that he was not receiving proper medical treatment, and declared a hunger strike to protest it. By the time he stopped it more than three weeks later, he said, he was left like “a skeleton walking, swaying, in its cell.”

In June 2022, Mr. Navalny was transferred to a harsher prison, where problems with his spine deteriorated, he said, because he had to spend most of his time with his movements constrained by the tight confines of the punishment cell. Doctors that came to see him did not disclose his diagnosis, he said, adding that he was also administered undisclosed shots.

In January 2023, Mr. Navalny’s wife, Yulia, said in a post on Instagram that her husband was sick with a high fever and that instead of helping him, prison officials transferred another sick person into his cell. The authorities refused to transfer him to a medical facility at the time, Mr. Navalny said, and a group of Russian doctors wrote letters demanding the authorities treat him.

For a time, his condition improved, but weeks later an ambulance had to be called because of an acute stomach condition, Mr. Navalny’s lawyer said. Last December, Mr. Navalny lost consciousness in his cell, his spokeswoman said.

“We don’t know what it was,” she said on social media. “But given that he is deprived of food and is kept in a punishment cell without ventilation, with minimum walks, it looks like a collapse from hunger.”

In December, Mr. Navalny spent almost three weeks traveling across the Russian prison system being transferred to his new penal colony in the Arctic. Upon arrival, he said that the trip was “pretty exhausting.”