A former Russian double agent, convicted of treason in Moscow for passing on the names of dozens of spies in Great Britain, was found unconscious on a park bench with his daughter after being exposed to what authorities say was an unidentified substance. 

Sergei Skripal, once a colonel in Russia's GRU military intelligence service, was discovered near a shopping mall in Salisbury.  Police say both Skripal and his daughter are in critical condition, suffering from an unknown illness.

Reuters:

Skripal, who passed the identity of dozens of spies to the MI6 foreign intelligence agency, was given refuge in Britain after he was exchanged in 2010 for Russian spies caught in the West as part of a Cold War-style spy swap at Vienna airport.

While the British authorities said there was no known risk to the public, police sealed off the area where the former spy was found and a pizza restaurant called Zizzi in the center of Salisbury.  Some investigators wore yellow chemical suits.

"We're speaking to witnesses, we're taking forensic samples at the scene, we're doing toxicology work and that will help us to get to an answer," Rowley told BBC radio.  He said counter-terrorism police were assisting the investigation.

"We have to remember: Russian exiles aren't immortal, they do all die and there can be a tendency to conspiracy theories.  But likewise we have to be alive to the fact of state threats," he said, pointing to the murder of Alexander Litvinenko.

With so little information so far, Skripal's illness should be seen as coincidence.  At the same time, it is impossible to avoid speculation that Russian intelligence carried out an assassination.  There have been literally dozens of suspicious deaths of critics of Vladmir Putin in the last twenty years, some on foreign soil.

Consider the case of Alexander Litvinenko:

A British inquiry said President Vladimir Putin probably approved the 2006 murder of ex-KGB agent Litvinenko with radioactive polonium-210 in London.  The Kremlin has repeatedly denied any involvement in the killing of Litvinenko.

Litvinenko, 43, an outspoken critic of Putin who fled Russia for Britain six years before he was poisoned, died after drinking green tea laced with the rare and very potent radioactive isotope at London's Millennium Hotel.

The inquiry discovered that the last people to see Litvinenko before he was poisoned were two former colleagues from the FSB, the successor agency to the KGB. 

Just since 2014, there have been 38 suspicious deaths of Putin critics.

New York Post:

They include 12 who were shot, stabbed or beaten to death.  Another six died in explosions.  Ten more died of supposedly natural causes.  One died of mysterious head injuries, one slipped in a public bath, another was hanged in his jail cell and one died while drinking coffee.  Six more were reported as unknown, the USA Today reported.

Two of the victims – Oleg Erovinkin and Alex Oronov – are linked to a dossier written by former British spy Christopher Steele that alleges Trump campaign officials colluded with the Kremlin during the 2016 presidential election, the report said.

Erovinkin, 61, a Russian intelligence officer, was found dead in the back of his car in Moscow on Dec. 26 of unknown causes.

Oronov, 69, a Ukranian-born businessman, died of unexplained circumstances on March 2.

He had arranged a meeting with Trump lawyer Michael Cohen, Trump associate Felix Sater and Andrii Artemenko, a member of Ukraine's parliament, to discuss a peace plan for Ukraine that would have given Putin control of Crimea.

Artemenko claims Oronov, a naturalized American citizen, died of the stress that came with hatching the plan.

"Unfortunately, his heart could not endure it. He died ... Friend, your death will not have been in vain, nor will the deaths of tens of thousands of Ukranians and Russians, Alex Oronov, during this wild, undeclared war!" Artemenko wrote on Facebook after Oronov's death.

Prior to 2014, there were more than a dozen deaths of anti-Putin journalists, human rights advocates, and politicians.

To repeat, it is likely that some of these deaths can be attributed to coincidence or random acts of violence.  But too many prominent Putin critics have met their end to pass at least some of them off as anything but political assassinations by a ruthless dictator.