Denis Voronenkov, a prominent critic of Russian president Vladimir Putin, was gunned down outside a hotel in Kiev.  His police-supplied bodyguard shot and wounded the attacker, who later died in police custody.

Voronenkov, a Russian parliamentarian who opposed the Russian annexation of Crimea and defected to Ukraine last year, told the Washington Post he feared for his life just days before he was murdered.

"For our personal safety, we can't let them know where we are," he said Monday evening as he sat with his wife for an interview with The Washington Post.

Less than 72 hours later, he was dead, shot twice in the head in broad daylight outside the same lobby bar. It was a particularly brazen assassination that recalled the post-Soviet gangland violence of the 1990s. His wife, dressed in black, sobbed as she stooped down to identify Voronenkov's body, which lay beneath a black tarp in a pool of blood.

Ukrainian President Petro Poroshenko, just hours later, called the attack an "act of state terrorism by Russia." As of Thursday evening, police had not identified the assailant, who died in police custody after being shot by Voronenkov's bodyguard. Dmitry Peskov, a spokesman for Russian President Vladimir Putin, called the accusation a "fabrication."

In the weeks before his death, Voronenkov, a former member of Russia's pliant Communist Party, had told friends he was being targeted. Hackers had been trying to pry into his Twitter account and his wife's email. He had received threatening text messages, and the police had recently assigned him a bodyguard. There were rumors he was under surveillance.

"It's a totally amoral system, and in its anger it may go to extreme measures," he said as he sat next to his wife, Maria Maksakova, a fellow parliamentarian who defected with him. "There's been a demonization of us. It's hard to say what will happen. The system has lost its mind. They say we are traitors in Russia."

He said he could return only "when Putin is gone."

At a time when the question of Russian influence dominates U.S. politics, Voronenkov's death will add further scrutiny to the extent, and potential lethality, of Russia's reach abroad. It remained unclear who might have wanted to kill Voronenkov – theories include Russian agents, Ukrainian nationalists or business interests – but the fact remains that he is just the latest Kremlin opponent to wind up dead.

The most famous among them include Alexander Litvinenko, the former Russian FSB agent who was poisoned with a radioactive isotope in London in 2006. Political opponents of the Kremlin in Moscow have also been targeted, including Boris Nemt­sov, the opposition politician who was gunned down in sight of the Kremlin in 2015.

The list of Putin enemies who have died under mysterious or violent circumstances is long.  It includes not only politicians, but journalists, human rights activists, and intellectuals.

It should be prominently noted that there is no physical evidence that Vladimir Putin is responsible for these deaths, nor is there any evidence that he ordered the assassinations to be carried out.  Indeed, the murder of Voronenkov could have been a robbery gone bad or an attack by someone mentally unstable.  Authorities are still trying to identify the gunman, so his possible links to the Russian president – if any – are unknown.

But there are far too many coincidences to ignore.  Voronenkov was in the middle of giving important testimony to Ukrainian authorities in connection with a treason case against former president Viktor Yanukovych.  Another Putin critic, Sergei Magnitsky, who was giving Russian prosecutors information about a massive tax fraud case involving Russian oligarchs, was beaten in prison after being arrested and denied medical care.  He later died of his injuries.

And to complete the circle, a lawyer for Magnitsky's family "fell" from his 4th-floor apartment in Moscow on Tuesday.  He was going to testify at a trial on Wednesday involving the tax fraud case.  He is currently in critical condition.

There may be no direct evidence that Putin ordered these hits, but it is highly suggestive and should be pursued further.  But by whom?  The pattern has been well established: cross Putin, and you end up dead.

In Putin's world, it is unhealthy to oppose the autocrat.