The suspected poisoning of Alexei Navalny, the Russian politician and opposition leader, is only the latest such incident to involve critics of the Kremlin.
There's a long history there of mysterious fates.
Let’s go through some of the notable cases, starting in 1978.
[Commander Jim Neville, of Scotland Yard's Anti-Terrorist Squad, saying:] “… certainly now we're regarding it as a murder enquiry, yes."
Georgi Markov was the center of the infamous "poison umbrella" case of the Cold War.
He was a Bulgarian writer who defected to the West, believed to be jabbed with a poisoned umbrella on London's Waterloo bridge.
[Unidentified newsreader, saying:] “ The holes which could have contained the poison had been coated with a slow-dissolving wax. The assassin could have fired the pellet from such an umbrella.”
In 2004, Viktor Yushchenko, then a Ukranian opposition leader, was also poisoned.
He said it happened during a dinner outside Kyiv.
He was campaigning for the presidential election on a pro-western ticket.
He survived, but was left disfigured. Doctors found that his body contained 1,000 times more dioxin than normal.
Two years later came the death of Alexander Litvinenko, an ex-KGB agent and outspoken critic of Vladmir Putin.
He died in 2006 after drinking green tea laced with polonium-210 - a rare and potent radioactive element - at London’s Millennium Hotel.
A British inquiry concluded that the killing was likely to have been authorized at the highest levels of power.
[Robert Owen, Litvinenko inquiry chairman, saying:] “I have further concluded that the FSB operation to kill Mr Litvinenko was probably approved by Mr Patrushev, then head of the FSB, and also by President Putin."
In 2012, the body of whistleblower Alexander Perepilichny was found near his luxury home, also outside London.
British police ruled out foul play despite evidence that traces of a rare and deadly poison were found in his stomach.
Another opposition activist, Vladimir Kara-Murza believes he was poisoned in 2015 and 2017.
A German laboratory later found elevated levels of mercury, copper, manganese and zinc.
"I have absolutely no doubt that this was an attempt to kill because when doctors tell you, you have a 5 percent chance to live after this. That's not how they scare you, that's how they kill you with such a percentage."
And of course, the case of Sergei Skripal in 2018 -- the former Russian double agent who passed secrets to British intelligence.
He and his daughter were taken to hospital in critical condition in the town of Salisbury, England, but survived.
They were poisoned by Novichok, a group of nerve agents developed by the Soviet military.
Russia denied any involvement in all of the incidents.