Several prominent critics of Kremlin policies - ex-spies, journalists and politicians - have been poisoned in the past two decades. In the UK, two Russian ex-secret service agents were targeted: Alexander Litvinenko fatally with radioactive polonium-210 in 2006, and Sergei Skripal with the toxic nerve agent Novichok in 2018. The Kremlin denied any involvement. Alexei Navalny, who has been physically attacked before, appears to be the latest victim. Yet much remains unclear. Mysterious poisonings involving Russians often remain mysterious - a distinct advantage for assassins, compared with say an old-fashioned shooting in the street. Prof Mark Galeotti, a Russia expert at the Royal United Services Institute, told the BBC that "poison has two characteristics: subtlety and theatricality". "It's so subtle that you can deny it, or make it harder to prove. And it takes time to work, there's all kinds of agony, and the poisoner can deny it with a sly wink, so everyone gets the hint." Thorn in Kremlin's side Alexei Navalny is Russia's best-known anti-corruption campaigner and opposition activist. His slick, hard-hitting videos on social media have drawn many millions of views, and made him a thorn in the side of the Kremlin. A victim poisoned before a long flight can be stuck in the air long enough for the assassin to make an easy getaway. Mr Navalny, 44, fell acutely ill on a flight from Tomsk in Siberia on 20 August - so ill that it had to be diverted to Omsk. Russian investigative reporter and Putin critic Anna Politkovskaya, shot dead in 2006, claimed to have been poisoned on a flight to the North Caucasus in 2004, when she felt sick and fainted. Similarly, a slow-acting poison - polonium-210 - killed Litvinenko excruciatingly and it was weeks before the rare toxin was identified. As an alpha-particle emitter its radiation was not detected by a Geiger counter. The two alleged Russian killers - state agents, according to the subsequent UK inquiry - had plenty of time to fly home unsuspected. Mr Navalny has accumulated many enemies in Russia, not just among supporters of President Vladimir Putin, whose United Russia party he labels "the party of crooks and thieves". Mr Putin was a secret service officer in the Soviet KGB before becoming president in 2000. Mr Galeotti says that in this case "the Russian state seems to have been caught off-balance, which implies it wasn't a centrally planned operation". "This suggests it was the act of a powerful Russian, but not necessarily the state." Nerve agent symptoms Now fighting for his life in Berlin's Charité hospital, Mr Navalny is in an induced coma, being treated for "poisoning with a substance from the group of cholinesterase inhibitors". The hospital says the specific toxin remains unknown - tests are being done to identify it. But the poison's effect - inhibition of the enzyme cholinesterase in the body - "was confirmed by multiple tests in independent laboratories". That is the effect of military nerve agents, such as sarin, VX or the even more toxic Novichok. They interfere with the brain's chemical signals to the muscles, causing spasms, shortness of breath, heart palpitations and collapse. Mr Navalny's spokeswoman Kira Yarmysh suspects that poison was slipped into the cup of black tea he drank at a Tomsk airport cafe. He had not eaten anything before the flight, she says. That ominously echoes the case of Litvinenko, who drank poisoned tea in a London hotel.