By Richard Pérez-Peña

New York Times News Service

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 by a national government, amid rampant speculation that Russia was responsible.

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 a meeting Wednesday of the government’s emergency security committee to discuss the matter.

“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 Skripal, 66, and his daughter, Yulia Skripal, 33, “were targeted specifically,” Rowley said. He refused to say what chemical was used or even whether investigators had identified it.

Time and again, foes of Putin have died suddenly in Britain, under suspicious circumstances. In the most notorious case, Alexander 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 intense 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 who were also scientists.

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.

On Sunday afternoon, he and his daughter became severely ill in the quiet cathedral town of Salisbury, England. 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.

The Kremlin has denied any involvement in the attack, and on Wednesday, Maria Zakharova, a spokeswoman for the Foreign Ministry, said that suggestions of Russian culpability were part of an orchestrated campaign to drive a wedge between Russia and Britain.

“Before it was clear what happened, the traditional speculation was being put about,” she said.

Skripal has lived quietly and openly for years in Salisbury. But in the past two years, his brother and his son both have died in what family members have called mysterious circumstances.