Aleksei A. Navalny portrayed himself as invincible, constantly utilizing his hallmark humor to recommend that President Vladimir V. Putin couldn’t break him, irrespective of how dire his circumstances grew to become in jail.

But behind the courageous face, the fact was plain to see. Since his incarceration in early 2021, Mr. Navalny, Russia’s most formidable opposition determine, and his workers usually instructed his circumstances had been so grim that he was being put to demise in sluggish movement.

Now his aides consider their fears have come true.

The explanation for Mr. Navalny’s demise in jail at 47 has not been established — in actual fact his household has not but even been allowed to see his physique — however Russia’s harshest penal colonies are recognized for hazardous circumstances, and Mr. Navalny was singled out for notably brutal therapy.

“Aleksei Navalny was subjected to torment and torture for three years,” the Russian journalist and Nobel Peace Prize winner Dmitri A. Muratov wrote in a column after his demise was introduced on Friday. “As Navalny’s doctor told me: the body cannot withstand this.”

More than 1 / 4 of Mr. Navalny’s incarceration since 2021 was spent in freezing “punishment cells” and he was typically denied entry to medical care. He was transferred to ever crueler prisons. And at one level, he stated he was being given injections however was prevented from discovering out what was within the syringes. His group frightened he was once more being poisoned.

What particularly led to Mr. Navalny’s demise on Friday at a distant jail above the Arctic Circle could stay a thriller. The Russian jail service launched an announcement Friday afternoon saying that Mr. Navalny felt sick and all of the sudden misplaced consciousness after being exterior.

Russian state media reported that he had suffered a blood clot. But the story modified on Saturday, when Mr. Navalny’s mom and lawyer arrived on the jail. They had been instructed he had suffered from “sudden death syndrome,” which appeared to point sudden cardiac arrest, in keeping with Ivan Zhdanov, director of Mr. Navalny’s anti-corruption basis.

Investigators instructed a lawyer for Mr. Navalny {that a} repeat examination was being performed and the outcomes could be launched subsequent week. Mr. Navalny’s workers referred to as for the physique to be launched instantly in order that his household may order an impartial evaluation, accusing Russian authorities of mendacity to hide the physique.

According to his aides, Mr. Navalny had been put in a punishment cell on the Arctic jail within the Yamalo-Nenets Autonomous Region on Wednesday, two days earlier than Russian authorities introduced his demise.

His spokeswoman, Kira Yarmysh, stated that marked his twenty seventh time in such an inhumane house, normally a roughly 7-feet-by-10-feet concrete cell with insufferable circumstances — chilly, damp and poorly ventilated. His newest spherical of punishment, had he survived, would have taken his complete interval in such a cell to 308 days, greater than 1 / 4 of his time in incarceration, in keeping with Ms. Yarmysh.

Once a day at 6:30 a.m., prisoners within the punishment cells on the Arctic facility are allowed right into a coffin-like concrete enclosure open to the sky by means of a metallic grate, Mr. Navalny stated in a message from the facility earlier this yr. It seemed to be after such a session on Friday that Mr. Navalny misplaced consciousness, in keeping with the Russian jail service’s account. It was about -20 Fahrenheit exterior.

In a letter from jail final month, Mr. Navalny described how he may stroll a complete of 11 steps from one finish of the open-air house to the opposite, noting that the coldest it had been up to now on one in all his walks was -26 Fahrenheit.

“Even at this temperature, you can walk for more than half an hour, so long as you have time to grow a new nose, ears and fingers,” he wrote. “There are few things as invigorating as a walk in Yamal at 6:30 in the morning. And what a wonderful fresh breeze blows into the yard, despite the concrete enclosure, wow!”

While strolling there on a latest day, he stated he was freezing and fascinated by how Leonardo DiCaprio climbed right into a lifeless horse to flee the chilly within the wilderness survival film “The Revenant.” A lifeless horse would freeze in that a part of Russia inside quarter-hour, Mr. Navalny surmised.

“Here we need an elephant — a hot, fried elephant,” he stated.

Mr. Navalny typically employed such wit within the face of his inhumane therapy. But it had turn out to be more and more clear, over his three years of incarceration, that he may not survive.

“The cumulative treatment of Navalny over several years in prison — in a way you could say it was driving him close to death,” Mariana Katzarova, the United Nations Human Rights Council particular rapporteur on the human rights state of affairs in Russia, stated in an interview Saturday. “We don’t know yet. We need an investigation.”

For a time, Mr. Navalny did appear nearly invincible.

In August 2020, he fell ailing on a flight from the Siberian metropolis of Tomsk to Moscow, after being poisoned with a nerve agent from the Russian-made Novichok household. He was put right into a medically induced coma for 2 weeks throughout therapy in Germany — and survived.

The U.S. authorities later attributed the poisoning to Russia’s Federal Security Service, often known as the F.S.B.

Despite the assassination try, Mr. Navalny returned to Russia in early 2021 to proceed his combat in opposition to Mr. Putin, who denied Russia’s involvement within the poisoning, and rapidly discovered himself imprisoned. His well being started to deteriorate nearly instantly.

In March 2021, he complained about extreme again ache that later become an issue along with his leg.

He demanded that jail authorities present him with correct medical care and provides him medicine. Instead, they subjected him to sleep deprivation, he stated. At the tip of March 2021, he declared a starvation strike over his therapy, and Russian medical doctors and Hollywood stars took up his trigger in open letters to Mr. Putin.

About three weeks later, Mr. Navalny was examined by an impartial panel of medical doctors. The checks by the medical doctors discovered that, “soon enough, there won’t be anyone to treat,” Mr. Navalny stated in a message posted to Instagram.

Last yr, Mr. Navalny wrote from jail that his jokes in regards to the punishment cell shouldn’t normalize the surroundings. He lamented {that a} fellow political prisoner, who had spoken out in opposition to the conflict in Ukraine, had been put in a punishment cell, regardless of being disabled and lacking a part of his lung.

Mr. Navalny described dire well being circumstances in jail, the place he said many inmates suffered from tuberculosis. He additionally complained early final yr in regards to the administration in his former jail putting a mentally unwell individual in a cell reverse his, as a type of torment, and an ailing prisoner in his small cell.

At the time, his lawyer, Vadim Kobzev, stated the jail intentionally contaminated him with a respiratory sickness, refused to provide him medication after which “treated” him with large doses of contraindicated antibiotics. Mr. Navalny suffered extreme abdomen ache and misplaced greater than 15 kilos consequently, Mr. Kobzev stated.

“These actions can’t be regarded as anything other than an open strategy to destroy Navalny’s health by any and all means,” Mr. Kobzev stated in an announcement on the time. “Obviously, the prison wouldn’t risk engaging in this level of demonstrative unlawfulness without approval from Moscow.”

Mr. Kobzev has since been arrested on extremism fees for associating with Mr. Navalny — a part of a broader roundup of the opposition chief’s attorneys late final yr.

Mr. Navalny suffered a dizzy spell and was placed on an IV drip in an unexplained medical episode in early December. But Russian authorities nonetheless transferred him later that month from a jail within the Vladimir area, about 130 miles east of Moscow, to the “special regime” penal colony within the Arctic the place he died.

Several medical doctors contacted after his demise, together with one who was concerned in his preliminary therapy within the Siberian metropolis of Omsk, stated his demise was possible unrelated to his poisoning greater than three years earlier, given his sturdy restoration.

But he confronted many different well being hazards since then.

“A Russian prison is a place where you have to be prepared to die every day,” Mikhail B. Khodorkovsky, a Russian tycoon who spent a decade in jail after difficult Mr. Putin, stated on Friday.

In the interview, Mr. Khodorkovsky, who was launched in 2013, stated a prisoner should discover a strategy to deal with the burden as a check in an effort to survive mentally, and Mr. Navalny had finished that. But even then, he added, “this will not protect you from being killed.”

Anton Troianovski contributed reporting.