Live Updates: Aleksei Navalny Is Dead, Russian Authorities Say

Alexei Navalny, surrounded by journalists, on high flight from Schoenfeld, Germany, to Moscow in 2021. He was arrested upon arrival in a Moscow airport.Credit…Mstyslav Chernov/Associated Press

In August 2020, soon after a private plane carrying Aleksei A. Navalny touched down in Berlin, doctors treating him at the prestigious Charité hospital there became so alarmed that they called in the German Army.

Mr. Navalny was not suffering from low blood sugar or even a standard detective-novel poison like arsenic or cyanide. It was, the German doctors suspected, something far more dangerous, requiring the attention of military chemical weapons specialists, German officials said.

Nearly two weeks later, the German government confirmed the doctors’ fears: Mr. Navalny had been poisoned with a military-grade nerve agent from the Novichok family, a potent class of chemical weapon developed by the Soviet Union that was used at least once before in recent years in an attack on a Kremlin enemy.

After waking from a medically induced coma, Mr. Navalny would go on to say that the poison had been planted in his underwear at his hotel sometime before he boarded a flight in Siberia. He had traveled to the region to help organize opposition candidates before local elections.

Russian officials denied that there was any evidence of poisoning, and a doctor in Omsk, the Siberian city where his plane had made an emergency landing, said that Mr. Navalny had suffered an “imbalance in carbohydrates,” possibly caused by low blood sugar. Mr. Navalny’s wife and personal doctor dismissed the diagnosis as ridiculous. Labs in Sweden, France and Germany later confirmed that he had been poisoned.

Novichok, a Soviet-era weapon invented for military use, was used against Sergei V. Skripal, a former Soviet spy, and his daughter in a 2018 attack in Salisbury, England, that the British government attributed to Russia’s military intelligence arm, the G.R.U.

In December 2020, four months after his poisoning, Mr. Navalny released a video of himself — posing as an aide to a senior Russian security official — that he said showed him phoning a Russian intelligence officer and duping him into confessing to a plot to kill the opposition leader. Russia’s domestic intelligence agency, the Federal Security Service called Mr. Navalny’s video a forgery, according to the RIA Novosti state news agency.

The next month, despite facing an all-but-certain prison sentence, Mr. Navalny returned to Russia after spending months in Germany recovering from the poisoning attempt. He was arrested upon arrival in a Moscow airport. Just over two weeks later, he was handed a prison sentence that sent him to a penal colony.