What do we know, and not know, about the death?
DEATH
Navalny was incarcerated at the IK-3 penal colony north of the Arctic Circle located in Kharp in the Yamalо-Nenets region about 1,900 km (1200 miles) northeast of Moscow.
His death was announced by the Federal Penitentiary Service of the Yamalo-Nenets Autonomous District at 2:19 pm Moscow time (1119 GMT) on Feb. 16.
"On Feb. 16, 2024, in penal colony number 3, convict Navalny A.A. felt unwell after a walk, almost immediately losing consciousness.
"The medical staff of the institution arrived immediately, and an ambulance team was called.
"All necessary resuscitation measures were carried out, which did not give positive results. Doctors of the ambulance stated the death of the convict. The causes of death are being established."
No more details about the death have been given by the penitentiary service or by medical officials.
'SUDDEN DEATH SYNDROME'
Navalny's wife Yulia, speaking in Munich just a few hours after the announcement, said that she did not know whether or not to believe the news due to what she said were the lies of Russia.
But on Saturday, Navalny's spokeswoman Kira Yarmysh, confirmed that Navalny was dead.
His 69-year-old mother, Lyudmila, had been given a notice of the death, Yarmysh said. The time of death in that notice was 2:17 p.m. local time (0917 GMT) on Feb. 16, Yarmysh said.
"When Alexei's lawyer and mother arrived at the colony this morning, they were told that the cause of Navalny's death was sudden death syndrome," Ivan Zhdanov, who directs Navalny's Anti-Corruption Foundation, said on Saturday.
"Sudden death syndrome" is a vague term for different cardiac syndromes that cause sudden cardiac arrest and death.
An unidentified source told state-controlled RT that Navalny died from a blood clot.
THE BODY
It was unclear where Navalny's body was.
His mother had been told that the body had been taken to Salekhard, the town near the prison complex but when she arrived at the morgue it was closed.
When contacted by Navalny's lawyer, the morgue said it did not have Navalny's body, Yarmysh said.
Later, they were told by officials that the body would not be handed over until the investigation was complete, though earlier they had been told that the investigation had discovered no traces of criminality.
"Right now we don't have access to the body and we don't know for sure where it is, and we demand that the Russian authorities immediately give Alexei's body to his family," Yarmysh said in an interview with Reuters on Saturday.
An employee at the morgue in Salekhard told Reuters that Navalny's body had not arrived.
Novaya Gazeta cited unidentified sources as saying that the body had been taken to the morgue at the Salekhard District Clinical Hospital.
The newspaper said that the body showed signs of bruising consistent with some sort of seizure as well as traces of heart massage attempts. The newspaper said it was unclear why his heart stopped.
Some Russian media reported that a special team of investigators had arrived from Moscow. It is unclear when the post-mortem will take place.
HEALTH ISSUES
Just the day before his death, Navalny was shown joking in court via a video link to court. In mirth, he requested a judge use some of his vast salary to top up his own account.
The online news outlet SOTA reported that the court session was convened after an "argument" with a prison officer who tried to confiscate Navalny's pen. Navalny wrote later on Thursday that he had been given 15 days in solitary confinement.
Russian prisons are brutal and his team had repeatedly raised fears that he would be harmed or killed.
He was repeatedly placed in punishment cells for various offences, including for his attitude.
Though he joked about the conditions, the Polar Wolf colony is considered to be one of the toughest prisons in Russia.
"It has not been colder than -32°C yet," Navalny said in January. "Nothing quite invigorates you like a walk in Yamal at 6:30 in the morning."
"Even at this temperature, you can walk for more than half an hour only if you manage to grow a new nose, new ears and new fingers."
He posted a picture of his walking yard - concrete walled, topped with metal bars, 11 steps long and three steps wide.
Navalny mentioned a scene in the 2015 film "The Revenant", in which Leonardo DiCaprio shelters in the carcass of a horse.
"I don't think that would have worked here. A dead horse would freeze in 15 minutes," Navalny said. "We need an elephant here, a hot elephant, a fried one."
Last year, he raised a little publicised problem that haunts Russia's prisons: a lack of teeth due to poor nutrition.
Less than a year ago, his team said he was grappling with severe stomach pain in jail.
