What are the chemical weapons at the centre of another attack blamed on the Kremlin, in the alleged poisoning of Alexei Navalny?
German authorities said on Tuesday there was "unequivocal" evidence that Kremlin critic Alexei Navalny was poisoned with a Novichok nerve agent.
Chancellor Angela Merkel said the attack amounted to "attempted murder".
Tests were carried out by a specialised military laboratory after Mr Navalny was airlifted to Berlin for treatment, having fallen ill on a domestic Russian flight on August 20.
Navalny's team blamed the poisoning on the Kremlin, claiming that someone spiked the opposition leader's tea.
This is the second attack linked to Novichok nerve agents, following the 2018 poisoning of the former Russian spy Sergei Skripal and his daughter in Salisbury, England.
While the Skripals survived, one woman later died after coming into contact with what British authorities said was a perfume bottle containing the chemical.
Russia has denied any involvement in the poisonings.
So what exactly are Novichok nerve agents? And why do experts say their use points to the Kremlin?
The 'military-grade' chemical agents banned under the 1997 Chemical Weapons Convention.
It is believed that Novichok agents can be five to ten times as deadly as sarin and VX.
"These chemicals were developed and investigated by the former Soviet Union, and from what we know in the public domain...Russia seems to have had more interest in this than anyone else," toxicology professor Alastair Hay told Euronews.
"Novichoks, like any nerve agents, can be administered by a variety of routes. You can swallow it, you can breathe it in, it can enter through your eyeballs, or through your skin, and they're toxic by any means of administration," Professor Hay said.
In 2018 British investigators said that Novichok chemicals had likely been smeared on the door handle of Sergei Skripal's home. This is the first recorded use of the nerve agent.
"I think Novichok chemicals were used against the Skripals because they had never been used before...and they [Russia] were hoping that it would be difficult to detect them," Professor Hay explained.
"You need pretty sophisticated equipment to detect them because they're potent in very small concentrations, so this means their concentration in the blood is very low. So the ability of most laboratories to find these is limited," he said.
"You can administer a tiny dose and that would probably kill someone. I think the intention was certainly in the Skripals' case and Mr Navalny's case, to kill."