Five European nations have accused Russia of using a deadly toxin taken from a South American frog to kill opposition figure Alexei Navalny two years ago.
In a joint statement, the United Kingdom, Sweden, France, Germany and the Netherlands said the presence of epibatidine had been conclusively confirmed in samples taken from Navalny's body.
The toxin, found in poison dart frogs in South America, is about 100 times more potent than morphine and is not found naturally in Russia.
Researchers believe the frogs acquire the toxin through their diet, but those raised in captivity do not produce any.
Russian authorities said Navalny died in prison from natural causes after going for a walk and becoming ill in 2024.
But Navalny's widow, Yulia Navalnaya, said last year that two independent labs had found that her husband was poisoned shortly before his death.
"These labs in two different countries reached the same conclusion: Alexei was killed. More specifically, he was poisoned," Ms Navalnaya said.
Following the release of the joint statement from the five nations, Ms Navalnaya said on social media that she had been "certain from the first day" that her husband had been poisoned, "but now there is proof".
The alleged use of the deadly frog toxin marks an expansion in the arsenal of novel chemical weapons suspected of being used by Russia.
These are some of the other instances where the Russian regime is accused of unleashing poison upon its enemies.
First alleged attempt on Alexei Nevalny
Alexei Navlany, who crusaded against official corruption and staged anti-Kremlin protests, was Vladimir Putin's fiercest foe.
It is believed Navlany had been poisoned before.
It occurred on a domestic flight in Russia in 2020 when he fell ill, stumbling into the aisle and collapsing.
Navalny's plane, headed to Moscow from Tomsk in Siberia, was forced to make an emergency landing in Omsk so he could be rushed to hospital.
He was flown to a Berlin hospital, where he was placed in an induced coma and eventually made a full recovery after learning to walk and speak again.
German chemical weapons experts said toxicology reports showed he was poisoned with the Soviet-era nerve agent Novichok.
Novichok is a highly toxic nerve agent that slows the heart, paralyses the muscles used for breathing and — if the dose is big enough — can lead to death by asphyxiation.
A smaller dose may result in seizures, neuromuscular weakness, liver failure and other damage.
More generally, nerve agents are highly toxic chemicals that can be either gas, aerosol or liquid, which poison the nervous system and disrupt bodily functions.
The Kremlin denied it was involved in poisoning Navalny and accused Germany of failing to provide evidence.
Initially, it was believed the poison was put in a cup of tea Navalny had at a cafe at the airport before boarding his flight, as that was the only thing he ingested that morning.
But subsequent investigations revealed the substance was likely applied to his underwear in his hotel room the night before he left for his flight.
Navalny released a recording of a phone call he said he made to a Russian state security operative, claiming he tricked the agent into disclosing details of a botched plot to kill him by placing poison in his underpants.
It was not the first time the deadly Novichok agent had been deployed against a critic of the Russian government.
The Salisbury poisoning
In 2018, a man and woman were found slumped unconscious on a public bench outside a shopping centre in the southern English city of Salisbury.
The pair found by a doctor and nurse passing by would turn out to be former Russian double agent Sergei Skripal and his daughter Yulia.
Skripal, 66, is a former colonel in Russian military intelligence who was arrested in 2004 in Moscow after it was found he had been working as a double agent for the British.
He was accused of passing on the identities of Russia's spies to the UK's Secret Intelligence Service MI6, in return for $US100,000.
Skripal was later freed in a prisoner swap, pardoned in exchange for 10 Russian agents.
He was granted asylum in the UK and lived openly under his own name, with his Salisbury address appearing in public records.
Speculation over how he was poisoned was rife in the UK tabloid media as he, his daughter and a police officer who investigated their home, Nick Bailey, received treatment in hospital.
Russian authorities denied any involvement, but within weeks the leaders of the United States, the UK, France and Germany concluded the Russian government was behind the attack, noting there was "no plausible alternative explanation".
Detectives found a high concentration of Novichok on the front door of Skripal's home.
Detectives initially named two Russian intelligence officers, Anatoliy Chepiga and Alexander Mishkin as suspects.
In 2021, they named a third, senior Russian agent, Denis Sergeev as the on-the-ground commander.
The Skripals and the police officer were able to make a full recovery.
But the attack had more grave, unintended consequences on innocent civilians.
About four months after the poisoning, mother-of-three Dawn Sturgess died from exposure to the poison after her partner found a counterfeit perfume bottle that Russian spies were believed to have used to smuggle the Novichok into the country.
Sturgess and her partner Charlie Rowley were found unconscious at their home, a few kilometres from the scene of a nerve agent attack on a former Russian spy and his daughter in March.
Sturgess died after doctors made the decision to turn off her life support, while Mr Rowley ended up recovering.
Police said the perfume bottle, which was found to contain Novichok, was discarded in the Skripal attack, only to later be found by Mr Rowley.
Mr Rowley told ITV News that his partner became sick 15 minutes after spraying the perfume on her wrists and he came into contact with it after some tipped on his hands when attaching the spray dispenser to the bottle, but washed his hands.
An inquiry by the British government found that Russian President Vladimir Putin authorised the assassination attempt on the Skripals with Novichok and there was a clear link between the use and discarding of Novichok and the death of Dawn Sturgess as well as the poisoning of Charlie Rowley.
The murder of Alexander Litvinenko
In 2006, ex-KGB officer and Kremlin critic Alexander Litvinenko died an agonising death after drinking green tea laced with polonium-210 at London's Millennium Hotel.
Hours after drinking the tea, he suffered from sickness and stomach pain.
He lost all of his hair and died three weeks later from a cardiac arrest caused by organ malfunction.
Polonium-210 is a highly radioactive and extremely toxic element that emits intense alpha radiation.
One of Britain's top nuclear scientists told an inquiry the tiniest amount of polonium would be fatal.
Due to the levels of radiation, the autopsy on Litvinenko's body has been described as one of the most dangerous ever undertaken.
On his deathbed, Litvinenko claimed that Putin had directly ordered his assassination.
A British inquiry found that Russian agents had killed Litvinenko, probably with Putin's approval, but the Kremlin denied any involvement.
