LONDON — Britain offers wealthy Russians many attractions: the great city of London, the bucolic countryside, exclusive schools, and a global financial hub.

But for some former spies and other foes of President Vladimir Putin, it has seemingly become lethal.

The latest victims near death's door are 66-year-old Sergei Skripal — a former colonel in Russia's military intelligence service, then a turncoat helping British agents — and his 33-year-old daughter Yulia. Both were found comatose on a public bench Sunday in the medieval English city of Salisbury.

British officials say they were exposed to a rare nerve agent of undetermined origin. Their prognosis is unclear; officials have not said if they have suffered irreversible damage.

Some in Britain say the nerve agent attack fits a pattern of suspicious Russian-related deaths in the U.K. and in the United States, and are calling for a high level police investigation into whether Britain has become a killing ground for the state-sanctioned elimination of foes of the Russian government.

The brazen assault has not been formally blamed on Russia, but it is raising hard questions on how to deal with the country's increasingly aggressive stance — even as officials in the U.S. are trying to determine how to respond to Russian interference in U.S. elections.

Several politicians, analysts and intelligence agencies believe the case of Skripal, who moved to Britain after he was freed in 2010, may prove to be the work of the Russian government, Russian organized crime groups, or a fluid alliance of the two.

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"Russian leaders seem to go out of their way to get rid of anybody that seems to be in their way, someone who's betrayed them, someone who's interrupting the money flow, and they don't seem to care about borders, they just go wherever they have to go to get their guy," said Joe Serio, the American author of "Investigating the Russian Mafia," who spent nearly ten years with the anti-organized crime unit of Moscow's police.

"Britain happens to be one of the central places where Russians flee. It's the gateway to the West, the seat of the language, the seat of the empire, the seat of major finance," he said.

Yvette Cooper, chairwoman of the Home Affairs committee in Parliament that reviews police and intelligence matters, and Ian Blair, former head of the Metropolitan Police that is spearheading the inquiry, both said this week that a string of unexplained deaths must be investigated in light of the latest attack. Cooper cited a BuzzFeed News investigation into 14 deaths that may have been the result of foul play.

There was also a chilling message from Moscow in the days after the attack on Skripal, delivered by a Russian state television news anchorman who warned potential double agents they should expect a shortened life span in Britain.

"Alcoholism, drug addiction, stress and depression are inevitable professional illnesses of a traitor resulting in heart attacks and even suicide," Kirill Kleimenov said.

He didn't mention nerve agents — or radioactive poisons, like the one used against former spy Alexander Litvinenko in 2006 — as other possible risk factors.

The Litvinenko case is the best documented. The former KGB agent who had defected to Britain and publicly criticized Putin died in November 2006, three weeks after drinking tea containing the radioactive isotope polonium-210.

Alexander Litvinenko at the Intensive Care Unit of University College Hospital in London on Nov. 20, 2006. Natasja Weitsz / Getty Images

Litvinenko died slowly, with the poison transforming him into a stick-thin figure wasting away on a hospital bed, and he blamed Putin shortly before he died. A decade later, a laborious public inquiry concluded he had been killed by Russia's security service, "probably" with Putin's approval.

Less clear is the 2013 demise of Boris Berezovsky, an affluent Russian businessman who moved to Britain in the early 2000s after breaking with Putin.

He was an outspoken critic of Putin's policies, and at times was allied with Litvinenko, until he was found dead on a bathroom floor at his home in southern England. He had a scarf around his neck, leading many to think he had taken his own life, but after an inquest the coroner concluded it was not possible to establish beyond a reasonable doubt whether the oligarch was killed or committed suicide.

Unusual deaths have also taken place outside of Britain. The 2015 death of former Putin aide Mikhail Lesin in a Washington hotel room was officially blamed by the District of Columbia's chief medical examiner on accidental injuries suffered after days of heavy drinking, but officials never explained how he got the blunt force injuries to his head and body.

The military-backed investigation of the Skripal case has transformed the pleasant English city of Salisbury into a major crime scene. Specialist police in oversize yellow hazardous material gear are searching for clues, and forensics tents have been erected over suspicious areas — including the gravesite of Skripal's son, who died last year.

The goals are to remove any vestiges of the nerve agent that might threaten the public, determine what specific nerve agent was used and — even more important — where it might have been manufactured.

That could go a long way toward determining if the crime indeed has Russian origins or if the early speculation is off base.

If the forensic evidence provides indisputable proof of Russian government involvement, Britain will have to make good on public vows to punish Russia made by Foreign Secretary Boris Johnson and other senior figures.

But the arsenal at Prime Minister Theresa May's disposal is limited, in part because promising relations between the two countries soured after the Litvinenko case and Russia's intervention in Ukraine.

"I'm not sure there are a lot of clear options for the U.K. government on this," said Robin Niblett, director of the Chatham House think tank. "Expulsion of diplomats. No Prince William at the World Cup in Russia. Some lobbying with the European Union, making political capital, saying this is why you need sanctions in place."