That drew a sharp rebuke from Putin's spokesman, who called the accusation "absurd." Throughout the years, the Kremlin has always dismissed the notion of political killings with scorn.

But Putin’s critics couldn’t help drawing parallels with the unexplained deaths of other Kremlin foes. "I have an impression — I hope it’s only an impression — that the practice of killing political opponents has started spreading in Russia," said Gennady Gudkov, a former parliamentarian and ex-security services officer, to the Moscow Times.

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Here are some outspoken critics of Putin who were killed or died mysteriously.

Boris Nemtsov, 2015

In the 1990s, Nemtsov was a political star of post-Soviet Russia’s "young reformers." He became deputy prime minister and was, for a while, seen as possible presidential material — but it was Putin who succeeded former president Boris Yeltsin in 2000. Nemtsov publicly supported the choice, but he grew increasingly critical as Putin rolled back civil liberties and was eventually pushed to the margins of Russian political life. Nemstov led massive street rallies in protest of the 2011 parliamentary election results and wrote reports on official corruption. He also was arrested several times as the Kremlin cracked down on opposition rallies. In Feb. 2015, just hours after urging the public to join a march against Russia's military involvement in Ukraine, Nemtsov was shot four times in the back by an unknown assailant within view of the Kremlin. Putin took "personal control" of the investigation into Nemtsov’s murder, but the killer remains at large.

Boris Berezovsky, 2013

A self-styled tycoon who become a fixture in Yeltsin's inner circle in the late 1990s, Berezovsky is believed to have been instrumental in Putin's rise to power (including a media campaign that smeared Nemtsov). But Berezovsky was unable to exert the influence under the new president he had hoped. His falling out with Putin led to his self-exile in the United Kingdom, where he vowed to bring down the president. He also accused the Kremlin of orchestrating the killing of Alexander Litvinenko, a former intelligence officer and whistleblower poisoned to death in 2009. Berezovsky was found dead inside a locked bathroom at his home in the United Kingdom, a noose around his neck, in what was at first deemed a suicide. However, the coroner’s office could not determine the cause of death.

Stanislav Markelov and Anastasia Baburova, 2009

Markelov was a human rights lawyer known for representing Chechen civilians in human rights cases again the Russian military. He also represented journalists who found themselves in legal trouble after writing articles critical of Putin, including Novaya Gazeta reporter Anna Politkovskaya, who was slain in 2006. Markelov was shot by a masked gunman near the Kremlin. Baburova, also a journalist from Novaya Gazeta, was fatally shot as she tried to help him. Russian authorities said a neo-Nazi group was behind the killings, and two members were convicted of the deaths.

Sergei Magnitsky, 2009

Lawyer Sergei Magnitsky died in police custody in November 2009 after allegedly being brutally beaten, then denied medical care. He had been working for British-American businessman William Browder to investigate a massive tax fraud case. Magnitsky was allegedly arrested after uncovering evidence suggesting that police officials were behind the fraud. In 2012, Magnitsky was posthumously convicted of tax evasion, and Browder lobbied the U.S. government to impose sanctions on those linked to his death. The sanctions bill bears his name and has since been applied to rights abusers in other cases.

Natalia Estemirova, 2009

Natalya Estemirova was a journalist who investigated abductions and murders that had become commonplace in Chechnya. There, pro-Russian security forces waged a brutal crackdown to weed out Islamic militants responsible for some of the country's worst terrorist attacks. Like fellow journalist Anna Politkovskaya, Estemirova reported on civilians who often got caught between these two violent forces. Estemirova was kidnapped outside her home, shot several times — including a point-blank shot in the head — and dumped in the nearby woods. Nobody has been convicted of her murder.

Anna Politkovskaya, 2006

Anna Politkovskaya was a Russian reporter for Novaya Gazeta whose book, "Putin's Russia," accused the Kremlin leader of turning the country into a police state. She wrote extensively about abuse in Chechnya, and once or twice appeared on radio shows in Moscow with me. She was shot at point-blank range in an elevator in her building. Five men were convicted of her murder, but the judge found that it was a contract killing, with $150,000 of the fee paid by a person whose identity was never discovered. Putin denied any Kremlin involvement in Politkovskaya's killing, saying that her "death in itself is more damaging to the current authorities both in Russia and the Chechen Republic ... than her activities."

Alexander Litvinenko, 2006

"Alexander Litvinenko was a former KGB agent who died three weeks after drinking a cup of tea" laced with deadly polonium-210 at a London hotel, as Business Insider wrote a year ago. "A British inquiry found that Litvinenko was poisoned by Russian agents Andrei Lugovoi and Dmitry Kovtun, who were acting on orders that had 'probably been approved'" by Putin. Russia refused to extradite them, and in 2015 the Russian president granted Lugovoi a medal for "services to the motherland." After leaving the Russian Federal Security Service, Litvinenko became a vocal critic of the agency, which was run by Putin, and later blamed the security service for orchestrating a series of apartment bombings in Russia in 1999 that left hundreds dead. Russia's invasion of Chechnya followed later that year — and with it, the rise to power of Putin. Berezovsky was suspected to be complicit in at least part of the plot to bring Putin to the Kremlin, but he later sought to implicate Putin for Litvinenko's killing. Litvinenko also accused Putin ordering the murder of Politkovskaya.

Sergei Yushenkov, 2003

The affable former army colonel was a favorite of parliamentary reporters in the early 1990s, when I was learning the trade for the Moscow Times. Sergei Yushenkov had just registered his Liberal Russia movement as a political party when he was gunned down outside his home in Moscow. Yushenkov was gathering evidence he believed proved that the Putin government was behind one of the apartment bombings in 1999.

Yuri Shchekochikhin, 2003

As a journalist and author who wrote about crime and corruption in the former Soviet Union when it was still very difficult to do so, Yuri Shchekochikhin once joined me on a police raid of crack houses in Philadelphia in 1988. He was investigating the 1999 apartment bombings for Novaya Gazeta when he contracted a mysterious illness in July 2003. He died suddenly, a few days before he was supposed to depart for the United States. His medical documents were deemed classified by Russian authorities.

Editor's note: This post originally contained two sentences describing Alexander Litvinenko that should have been attributed to Business Insider. That attribution has been added.