Vladimir Putin stands with a gun at a shooting gallery of the new GRU military intelligence headquarters building as he visits it in Moscow November 8, 2006. Reuters

Ten years ago today, Alexander Litvinenko died from radiation poisoning in a hospital bed in London.

Litvinenko, a defector from the Russia's intelligence service the FSB, was a prominent critic of Putin and the Kremlin.

He died from ingesting a dose of the radioactive material polonium-210, which a British inquiry in 2016 found was most likely a political assassination approved by the head of the FSB and Russian President Vladimir Putin.

Litvinenko's death is one among a series of mysterious killings. Here are some of the other people Putin is suspected of assassinating:

Mikhail Lesin

The Washington DC medical examiner's office has confirmed that former Russian press minister Mikhail Lesin died of "blunt force trauma to the head."

Lesin, who founded the English-language television network Russia Today (RT) was found dead in a Washington, DC, hotel room in November 2015.

The Daily Beast reports that before his death, Lesin was considering making a deal with the FBI to protect himself from corruption charges.

For years, Lesin had been at the heart of political life in Russia and would have known a lot about the inner workings of the rich and powerful.

Anna Politkovskaya

Anna Politkovskaya was a Russian journalist who was critical of Putin. In her book "Putin's Russia," she accused Putin of turning his country into a police state. She was murdered by contract killers who shot her at point blank range in the lift outside her flat.

Five men were convicted of her murder, but the judge found that it was a contract killing, with $150,000 paid by "a person unknown."

A picture of slain journalist Anna Politkovskaya is shown during a candlelight vigil in front of the Russian Embassy. Mark Wilson/Getty Images

Natalia Estemirova

Natalia Estemirova was a journalist who sometimes worked with Politkovskaya.

She specialized in uncovering human-rights abuses carried out by the Russian state in Chechnya.

She was abducted from outside her home and later found in nearby woodland with gunshot wounds to her head. No one has been convicted of her murder.

Stanislav Markelov and Anastasia Baburova

Human-rights lawyer Stanislav Markelov represented Politkovskaya and other journalists who had been critical of Putin.

He was shot by a masked gunman near the Kremlin. Journalist Anastasia Baburova, who was walking with him, was also shot when she tried to help him.

Boris Nemtsov speaks at a news conference on "Corruption and Abuse in Sochi Olympics." Alex Wong/Getty Images

Boris Nemtsov

Boris Nemtsov was a former deputy prime minister of Russia under Boris Yeltsin who went on to become a big critic of Putin — accusing him of being in the pay of oligarchs.

He was shot four times in the back just yards from the Kremlin as he walked home from a restaurant. Despite Putin taking "personal control" of the investigation into Nemtsov's murder, the killer has not been found.

Boris Berezovsky

Boris Berezovsky was a Russian oligarch who fled to Britain after he fell out with Putin. During his exile he threatened to bring down Putin by force. He was found dead at his Berkshire home in March 2013 in an apparent suicide, although an inquest into his death recorded an open verdict.

Berezovsky was found dead inside a locked bathroom with a ligature around his neck. The coroner couldn't explain how he had died.

The British police had on several occasions investigated alleged assassination attempts against him.

Boris Berezovsky wears a mask showing the face of Russia's President Vladimir Putin, as he leaves Bow Street Magistrates Court. Graeme Robertson/Getty Images

Paul Klebnikov

Paul Klebnikov was the chief editor of the Russian edition of Forbes. He had written about corruption and dug into the lives of wealthy Russians.

He was killed in a drive-by shooting in an apparent contract killing.

Sergei Yushenkov

Sergei Yushenkov was a Russian politician who was attempting to prove the Russian state was behind the bombing of an apartment block.

He was killed in an assassination by a single shot to the chest just hours after his political organization, Liberal Russia, had been recognized by the Justice Ministry as a party.

An original version of this post was written by Jeremy Wilson.