If a former Russian double-agent and his daughter were poisoned by foreign agents in the U.K., it would mark a “very cruel†shift toward punishing the families of defectors, an expert in Russian espionage says.

Sergei Skripal, 66, and his 33-year-old daughter, Yulia, were discovered in critical condition Sunday on a public bench in Salisbury, eight years after Skripal was freed in a spy swap between the U.S. and Russia. U.K. police announced Wednesday that the two were affected by a nerve agent in what’s being treated as a “targeted†incident.

Many have compared the case to the fatal poisoning of Alexander Litvinenko, another former Russian agent who defected to Britain. The U.K. has promised swift retribution if it determines that Russia is behind the poisoning.

The Kremlin has denied involvement, calling Skripal’s plight “an anti-Russian campaign.â€

Russian-American author Yuri Felshtinsky, who co-authored the book “Blowing Up Russia†with Litvinenko in 2002, says Skripal’s situation points strongly toward another Russian poisoning plot. However, such an attack would be unique in that Russia doesn’t typically seek retribution against the children of defectors.

“If they have a chance to find a defector, they kill him,†Felshtinsky told Â鶹´«Ã½ Channel on Wednesday. “But you never touch children.â€

He says this has been standard practice for Russia’s Federal Security Service (FSB) for years, just as it was for the agency’s Soviet Union-era predecessor, the KGB.

Skripal’s son died last year in Russia, and his wife passed away at the age of 59 in 2012. Felshtinsky says both deaths occurred under “questionable circumstances,†with each case involving contradictory causes of death. Now, with Skripal and his daughter critically ill and facing death, Felshtinsky says the signs point strongly to a more vindictive shift in how Russia punishes traitorous spies.

“This is very cruel, and this may indicate a change in the rules of the game,†Felshtinsky said. “People who are defecting may have to be afraid for their family as well.â€

He says the last time Russian agents targeted someone’s family was in 1940, when Soviet agents staged a failed assassination attempt on exiled politician Leon Trotsky. The attackers shot Trotsky’s grandson in the foot and abducted his assistant, whom they later killed. Trotsky himself was fatally wounded in another assassination attempt three months later.

The spy swap

Skripal was a former colonel in Russia’s GRU military intelligence service who was convicted of exposing Russian spies to British intelligence in 2006.

Felshtinsky says Skripal’s situation is complicated, because he was the first Russian citizen handed over by Russia to a foreign power in exchange for Russians caught spying abroad. Skripal was among four prisoners released in 2010 as part of a swap for 10 Russian agents held by the U.S., including model Anna Chapman. Chapman had been part of a Russian sleeper cell in the United States.

Russia would have preferred to offer up foreign-born agents in the swap, but it didn’t have any in custody at the time, Felshtinsky said. Instead, Skripal and three other Russian double agents were used in the trade.

“They let him go, but decided to kill him when the opportunity presented itself,†he said.

Skripal went into hiding in Britain after the exchange, while Chapman returned to Russia and became a TV presenter and advocate for Vladimir Putin’s government. The 36-year-old continues to promote the Russian government through various media, including on her Instagram page, where she posts racy photos accompanied by pro-Russian messages.

 

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Poisoning

Many have drawn comparisons between Skripal’s situation and the death of Litvinenko, who died of radiation poisoning in 2006 from exposure to polonium 210. Litvinenko was a former FSB agent who sought political asylum in Britain, while Skripal was an officer in the Russian military who passed information to British intelligence.

Felshtinsky says it would have been “kind of stupid†for Russia to use radiation poisoning again. He says it’s a very deadly tactic, but also an easy one to trace. “The moment you understand what poison was used, you may actually establish how the poison was travelling to London.â€

Felshtinsky says the poison was likely delivered by a gas or spray. “Let’s hope they survive – although I think are hopes are very low now – and then they may tell us the story,†Felshtinsky said.

Police have confirmed that a nerve agent was used in the attack, but they did not indicate whether it was sarin or VX nerve gas.

One of the first officers on the scene in Salisbury has also been affected by the gas, Assistant Commissioner Mark Rowley, of the Metropolitan Police Service, said at a briefing Wednesday.

“We are not seeing any evidence of a widespread health risk,†he said.

A VX nerve agent was used last year at a Kuala Lumpur airport to fatally poison Kim Jong Nam, brother to North Korean leader Kim Jong Un.