Sergei Skripal collapse draws attention to poisonings as assassination method
Deadly, silent and potentially undetectable, poisons appeal to assassins.
With mystery surrounding the circumstances of ex-spy Sergei Skripal's collapse following suspected exposure to an unknown substance, attention has once again been drawn to poisonings linked to Russia.
It has even been alleged that a Russian hitman is on the loose in the UK following the death of Alexander Perepilichnyy.
Yuri Felshtinsky, a former associate of Alexander Litvinenko, said: "Poisoning is the method of choice for the FSB.
"In the context of the Russian presidential election, this has all the hallmarks of a Putin assassination.
"He is warning anyone in the FSB never to defect as they'll be hunted down and killed."
He added: "The underlying atmosphere of terror in Russia means that even mere speculation serves its purpose - even if there was a different cause here.
"As in this case Sergei Skripal was a colonel in the FSB, like Alexander Litvinenko. The FSB always kills defectors as a loyalty warning to its agents."
Perhaps the most high-profile case of fatal poisoning is that of former Russian secret agent Mr Litvinenko.
The fierce Kremlin critic died in a London hospital in November 2006 from a fatal dose of the extremely rare radioactive isotope polonium-210, which was slipped into his drink at a London hotel.
The Russian authorities have refused to extradite the prime suspects in the murder, businessman Andrei Lugovoy and Dmitry Kovtun, who insist they are innocent.
Polonium-210 also leaves a radioactive trace, which in the Litvinenko case led investigators to Lugovoy.
In 2016 a public inquiry concluded that the killing of Mr Litvinenko had "probably" been carried out with the approval of Russian president Vladimir Putin.
Russian expat Mr Perepilichnyy, 44, had eaten his favourite sorrel soup prepared by his wife before he collapsed while out jogging near his home in Weybridge, Surrey, in November 2012.
An inquest into his death heard an experienced assassin could have poisoned the Russian whistleblower, even though extensive tests failed to identify the toxin.
Bill Browder, the London-based human rights campaigner and professed "number one enemy" of Mr Putin, said Mr Perepilichnyy had told him he had been receiving death threats, "making it reasonably likely that a Russian assassin was on the loose in the UK".
At the Old Bailey inquest, forensic toxicology specialist Dr Fiona Perry was asked if specially made rare poisons could be designed as a "warfare agent".
She said it depended on what access people had to them, adding that it may be easy for them if, for example, they had a plant growing in their garden.
Peter Skelton QC, counsel for the coroner, suggested some poisons were "impossible to detect" and could be deployed by an "experienced assassin".
The inquest is due to resume in April, and a coroner is yet to make a ruling.
In September 1978, Bulgarian dissident Georgi Markov was assassinated in London using a poison-loaded umbrella.
The murder involved a minuscule dose of ricin, a toxin produced naturally by the castor bean plant.
KGB agents and senior members of Bulgaria's secret police were suspected of being involved in the killing, but the communist defector's killers have never been brought to justice.
Doctors treating the Bulgarian dissident were initially baffled by his symptoms and suspected he had an especially serious case of blood poisoning.