(CNN) Russian opposition leader Alexey Navalny has long acknowledged the risks of confronting the Kremlin: The anti-corruption campaigner has been repeatedly jailed and spent long stretches in custody for organizing political protests.
But news that Navalny had fallen gravely ill on Thursday from a suspected poisoning sent a fresh shockwave through Russian society. While doctors have yet to rule conclusively about the cause of his illness, the possibility that he was poisoned raises worrying parallels with some of the more brazen political killings in Russia's recent past.
For an international audience, two cases may come to mind: The murder of former Russian agent Alexander Litvinenko, who was poisoned in London in 2006, and the 2018 Salisbury nerve-agent attack.
Both cases involved exotic poisons and targeted individuals who were seen as an irritant to the Kremlin. The substance used to kill Litvinenko was a rare radioactive isotope, polonium-210. In Salisbury, former Russian agent Sergei Skripal and his daughter Yulia were sickened by Novichok, a military-grade nerve agent. A British couple, Dawn Sturgess and Charlie Rowley, were also exposed to the agent; Sturgess subsequently died.
The Kremlin has consistently denied involvement in both of those high-profile attacks. But Western governments, independent researchers and Russia-watchers see a consistent pattern of Russian state involvement in assassinations both inside Russia and abroad.
Before Navalny's rise to prominence, the most visible leader of the Russian opposition was Boris Nemtsov, a former deputy prime minister. In 2015, Nemtsov was gunned down on a bridge within sight of the Kremlin. That killing drew worldwide condemnation, and while a court sentenced five Chechen men to lengthy prison terms over the murder, Nemtsov's supporters maintain that those ultimately responsible for ordering his assassination escaped justice.
Navalny has faced physical threats as well. In 2017, he lost vision in one of his eyes after he was attacked with an antiseptic green dye. Last year he was hospitalized after he was affected by an unknown chemical while in police custody. He has spoken openly with the press about the possibility of assassination.
But the opposition leader's possible poisoning on Thursday has at first glance some parallels with another prominent case: The 2004 poisoning of leading Russian investigative journalist Anna Politkovskaya.
Navalny started feeling sick, his spokesperson Kira Yarmysh said, while on a return flight to Moscow from the Siberian city of Tomsk, forcing an emergency landing in Omsk. Navalny drank black tea in an airport cafe before takeoff, she said, and the airline said he had no drinks while on board.
That's reminiscent of the incident in 2004, when Politkovskaya was also sickened while flying to southern Russia amid a hostage crisis in Beslan, in Russia's North Ossetia region. Her editor told the Committee to Protect Journalists at the time that Politkovskaya had not eaten anything before the trip and only drank tea on the plane.
Politkovskaya survived that poisoning but was assassinated in her stairwell in 2006. At the time, she had been deeply involved in researching human-rights violations in Russia's campaign against insurgents in the north Caucasus.
Navalny is well known as an activist, but it's his investigations that have been the biggest thorn in the side of some of Russia's powerful people. His investigative videos about the apparent unexplained wealth of top government officials has particularly raised the ire of the Kremlin. One video about former Russian Prime Minister Dmitry Medvedev drew more than 35 million views on YouTube.
More recently, Navalny has tangled with Yevgeny Prigozhin, a Kremlin-linked oligarch who is under US sanction over interference in American elections. Navalny said he recently decided to shut his investigative non-profit after a company affiliated with Prigozhin filed a major lawsuit against him and his foundation.
The Kremlin routinely dismisses Navalny's allegations about widespread government corruption, and Navalny himself is barred from running for political office over his criminal conviction in a fraud case -- a conviction the European Court of Human Rights (ECHR) ruled was politically motivated.
But the apparent poisoning of Navalny also comes as Russian civil society is transfixed by events next door: The massive protests that have swept Belarus over apparent fraud in the re-election of strongman Alexander Lukashenko.
In recent days, Navalny has given prominent coverage to events in Belarus in his online programs, and has spoken forcefully in support of the Belarusian opposition. In his view, it's clear that Lukashenko's confrontation with his own people is a foreshadowing of a potentially bigger struggle: Some future showdown between Putin and a motivated, well-organized and massive opposition.