LONDON — A former Russian spy and his daughter were poisoned by a nerve agent in Britain this week, British police said Wednesday, heightening suspicions that the episode was an assassination attempt ordered by Russia.
The development forces the British government to confront the possibility that once again, an attack on British soil was carried out by the government of President Vladimir Putin, which Western intelligence officials say has, with alarming frequency, ordered the killing of people who have crossed it.
Prime Minister Theresa May and her Cabinet ministers held an emergency security meeting Wednesday.
“This is being treated as a major incident involving attempted murder by administration of a nerve agent,” said Mark Rowley, Britain’s chief police official for counterterrorism and international security.
The former spy, Sergei V. Skripal, 66, and his daughter, Yulia Skripal, 33, “were targeted specifically” in the attack in the English town of Salisbury, Rowley said.
Rowley refused to say what chemical was used, or even whether investigators had identified it. Forensic investigators were still examining the scene Wednesday.
On Sunday afternoon, the Russians became severely ill at a mall in Salisbury, a quiet cathedral town. They lost consciousness and remain in critical condition.
Some of the emergency workers who went to the scene also became ill, and one police officer has been hospitalized in serious condition, Rowley said.
Time and again, foes of Putin have died suddenly in Britain, under suspicious circumstances. In the most notorious case, Alexander V. Litvinenko, a former Russian agent who was harshly critical of Putin, was fatally poisoned in 2006 with a rare radioactive metal, and an inquiry later concluded that he was assassinated by Russian operatives, probably with Putin’s approval.
“I’m sure that in some of these cases, there is a relatively natural explanation, but it is beyond the bounds of probability that they all are,” said James Nixey, manager of the Russia and Eurasia program at Chatham House, a British foreign affairs think tank.
The British government has been accused of being less than eager to get to the bottom of those deaths, or to hold anyone responsible, but political and security analysts say this time is likely to be different.
Given the government’s sensitivity to that criticism, and the worldwide attention on the Skripal case, a thorough investigation is probably unavoidable, they say, and if Russian involvement is found, an aggressive response may be inevitable, too.
The resources and expertise involved in producing and using a nerve agent suggest the involvement of a military or intelligence agency, as in two highly publicized episodes last year: Syrian government forces used sarin gas, a nerve agent, against a rebel-held village; and the North Korean government is believed to have been behind the assassination of the half brother of the country’s leader using another nerve agent, VX.
“We can’t say for sure right now, but the more sophisticated and the rarer the poison, the more likely it is to come from the Russian state or elements within it,” said Ben Judah, a biographer of Putin who has also researched the lives of Russian expatriates in Britain.
But the evidence of state sponsorship is not conclusive, security and political analysts said. In 1995, a religious cult killed 12 people in the Tokyo subway by releasing sarin, made by some of its adherents.
Experts also cautioned that even when evidence points to Moscow, it is hard to determine whether the attacks were ordered by Russian oligarchs or organized crime bosses whose interests are aligned with the Putin government’s, elements within Russian intelligence acting on their own, or the Kremlin itself.
“Certainly, a nerve agent is not something an ordinary person can get their hands on,” said Vladimir Ashurkov, a Russian dissident living in Britain who is allied with the Russian opposition leader Alexei Navalny.
“But whether it’s sanctioned by the state,” he said, is still unproved. “It may have been a decision from Mr. Skripal’s colleagues who he betrayed rather than from the highest circles of Russian power.”
In 2006, a Russian court convicted Skripal, a former colonel in Russia’s military intelligence, of selling secrets to the British. In 2010, he was released from prison and sent to Britain as part of an exchange of imprisoned spies.
The Kremlin has denied any involvement in the attack.