■Putin’s political critics have been attacked or killed in various ways.

■Assassination attempts against Putin’s foes have been common.

■Reports of prominent Russian executives dying under mysterious circumstances.

TALLINN, Estonia — The attacks range from the exotic — poisoned by drinking polonium-laced tea or touching a deadly nerve agent — to the more mundane of getting shot at close range. Some take a fatal plunge from an open window.

Over the years, Kremlin political critics, turncoat spies and investigative journalists have been killed or assaulted in a variety of ways.

On Friday, Russian authorities said President Vladimir Putin’s key political challenger, Alexei Navalny, died in an Arctic prison colony. The details of what happened are unknown; Navalny’s team says it has no official confirmation of his death and Russian authorities say they are establishing why he died. His allies previously accused Russian officials of trying to poison him with a nerve agent in 2020.

Assassination attempts against foes of Putin have been common during his nearly quarter century in power. Those close to the victims and the few survivors have blamed Russian authorities, but the Kremlin has routinely denied involvement.

There also have been reports of prominent Russian executives dying under mysterious circumstances, including falling from windows, although whether they were deliberate killings or suicides is sometimes difficult to determine.

Some Prominent Cases of Documented Killings or Attempted Killings

In August 2020, Navalny fell ill on a flight from Siberia to Moscow. The plane landed in the city of Omsk, where Navalny was hospitalized in a coma. Two days later, he was airlifted to Berlin, where he recovered.

His allies almost immediately said he was poisoned, but Russian officials denied it. Labs in Germany, France and Sweden confirmed Navalny was poisoned by a Soviet-era nerve agent known as Novichok, which he reported had been applied to his underwear.

Navalny returned to Russia and was convicted last August of extremism and sentenced to 19 years in prison. It was his third conviction with a prison sentence in two years. He said the charges were politically motivated.

On Friday, Russia’s Federal Penitentiary Service said Navalny felt unwell after a walk and lost consciousness. An ambulance arrived, but he could not be revived. The service said his cause of death was “being established.”

In 2018, Pyotr Verzilov, a founder of the protest group Pussy Riot, fell severely ill and also was flown to Berlin, where doctors said poisoning was “highly plausible.” He eventually recovered. Earlier that year, Verzilov embarrassed the Kremlin by running onto the field during soccer’s World Cup final in Moscow with three other activists to protest police brutality. His allies said he could have been targeted because of his activism.

Prominent opposition figure Vladimir Kara-Murza survived what he believes were attempts to poison him in 2015 and 2017. He nearly died from kidney failure in the first instance and suspects poisoning but no cause was determined. He was hospitalized with a similar illness in 2017 and put into a medically induced coma. His wife said doctors confirmed he was poisoned.

Kara-Murza survived, and his lawyer says police have refused to investigate. Last year, he was convicted of treason and sentenced to 25 years in prison. In January he was moved to a prison in Siberia and placed in solitary confinement over an alleged minor infraction.

The highest profile killing of a political opponent in recent years was that of Boris Nemtsov. Once deputy prime minister under Boris Yeltsin, Nemtsov was a popular politician and harsh critic of Putin. On a cold February night in 2015, he was gunned down by assailants on a bridge adjacent to the Kremlin as he walked with his girlfriend in a death that shocked the country.

Five men from the Russian region of Chechnya were convicted for his killing, with the gunman receiving up to 20 years. But Nemtsov’s allies said that was an attempt to shift blame from the government.

Former Intelligence Operatives

In 2006, Russian defector Alexander Litvinenko, a former agent for the KGB and its post-Soviet successor agency, the FSB, became violently ill in London after drinking tea laced with radioactive polonium-210. He died three weeks later.

Litvinenko had been investigating the shooting death of Russian journalist Anna Politkovskaya as well as the Russian intelligence service’s alleged links to organized crime. Before dying, Litvinenko told journalists the FSB was still operating a poisons laboratory dating from the Soviet era.

A British inquiry found that Russian agents had killed Litvinenko, probably with Putin’s approval, but the Kremlin denied any involvement.

Another former Russian intelligence officer, Sergei Skripal, was poisoned in Britain in 2018. He and his adult daughter Yulia fell ill in the city of Salisbury and spent weeks in critical condition. They survived, but the attack later claimed the life of a British woman and left a man and a police officer seriously ill.

Authorities said they both were poisoned with the military grade nerve agent Novichok. Britain blamed Russian intelligence, but Moscow denied any role. Putin called Skripal, a double agent for Britain during his espionage career, a “scumbag” of no interest to the Kremlin because he was tried in Russia and exchanged in a spy swap in 2010.

Journalists

Numerous journalists critical of authorities in Russia have been killed or suffered mysterious deaths, which their colleagues in some cases blamed on someone in the political hierarchy. In other cases, the reported reluctance by authorities to investigate raised suspicions.

Politkovskaya, the journalist for the newspaper Novaya Gazeta whose death Litvinenko was investigating, was shot and killed in the elevator of her Moscow apartment building on Oct. 7, 2006 — Putin’s birthday. She had won international acclaim for her reporting on human rights abuses in Chechnya. The gunman, from Chechnya, was convicted of the killing and sentenced to 20 years in prison. Four other Chechens were given shorter prison terms for their involvement in the murder.

Yuri Shchekochikhin, another Novaya Gazeta reporter, died of a sudden and violent illness in 2003. Shchekochikhin was investigating corrupt business deals and the possible role of Russian security services in the 1999 apartment house bombings blamed on Chechen insurgents. His colleagues insisted that he was poisoned and accused the authorities of deliberately hindering the investigation.

Yevgeny Prigozhin and His Lieutenants

A plane crash last August which killed Yevgeny Prigozhin and top lieutenants of his Wagner private military company came two months to the day after he launched an armed rebellion that Putin labeled “a stab in the back” and “treason.” While not critical of Putin, Prigozhin slammed the Russian military leadership and questioned the motives for going to war in Ukraine.

A U.S. intelligence assessment found that the crash that killed all 10 people aboard was intentionally caused by an explosion, according to U.S. and Western officials. The officials spoke on condition of anonymity because they were not authorized to comment. One said the explosion fell in line with Putin’s “long history of trying to silence his critics.”

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Putin’s spokesperson, Dmitry Peskov, rejected allegations the Kremlin was behind the crash. “Of course, in the West those speculations are put out under a certain angle, and all of it is a complete lie,” he told reporters.

In his first public comments after the crash, Putin appeared to hint there was no bad blood between him and Prigozhin. But former Kremlin speechwriter turned political analyst Abbas Gallyamov said: “Putin has demonstrated that if you fail to obey him without question, he will dispose of you without mercy, like an enemy, even if you are formally a patriot.”