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 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.

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On Saturday, military personnel removed an ambulance that may have been contaminated in the poisoning of the former Russian spy, CBS News' Roxana Saberi reports.

British Home Secretary Amber Ruud said Saturday the investigation is making progress.

Police officers dress in protective suits and breathing apparatus in London Road cemetery as they continue investigations into the poisoning of Sergei Skripal who was found critically ill on a bench in Salisbury with his daughter sparking a major incident on March 10, 2018 in Salisbury, England. Chris J. Ratcliffe / Getty

"There's over 200 witnesses involved and there's over 240 pieces of evidence," Ruud said. "So we need to give the police and all the investigative parts around them the space to get on with that."

British police are reviewing hours of CCTV footage from Sunday, including new images that show what appears to be a man and a younger woman walking together as another woman with a dog passes them. Minutes later, a police car speeds by toward the area where Skripal and his daughter Yulia were found on a park bench unconscious.

The daughter's childhood friend, Irina Petrova, said she remembers the victims as the "perfect family." She told BBC News that Yulia didn't do "anything to deserve" being targeted and said it seems like friends of the family are afraid to speak out.

"I'm starting to get scared," Petrova told BBC News. "No one wants to speak -- even her relatives."

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 the Russian government, but it is raising hard questions on how to deal with Russia'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.

"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 10 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.

"No attempt on an innocent life on British soil should go uninvestigated or unpunished," said Cooper, who called for the National Crime Agency to scrutinize the 14 deaths in light of reports that U.S. intelligence officials believe they may be linked to Russia — even though British police have not reached the same conclusion in most cases.

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.

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.

There are also serious doubts about the 2012 death of Alexander Perepilichny, a Russian businessman who provided vital testimony against Russian officials accused of stealing $230 million from a London hedge fund.

He died near his rented home while jogging. Two autopsies were inconclusive, with no obvious signs of foul play, but colleagues suspect he may have been poisoned with a rare, difficult-to-detect plant. A coroner's inquest is underway but no cause of death has been established.

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 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.

Serio, who worked with Moscow police, said it may turn out that the Russian government was not directly involved.

"I've seen many dramatic situations like these where, after the dust settles, it comes down to business interests, whether that business is contracts and money or information and spying," he said. "I'd start with Skripal's activities. Was he involved in business, private security, intelligence-related matters? Was he doing private consulting for companies? It's either betrayal, politics, or you're messing with my money," he said. "These guys are relentless."

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."

He does not believe British police and courts have been lax. Instead, he said the Russians have been effective in a crackdown on former agents who cooperate with British intelligence.

"The Russian have been very good at covering their tracks," he said. "A lot of this stuff is really difficult to prove. And if you don't have clear evidence, what's the point of going into court?