“This is a form of soft war that Russia is now waging against the West,” said Tom Tugendhat, chairman of the Foreign Affairs Committee in the British Parliament.
Mr. Tugendhat said that Britain should consider revoking the broadcast license of RT, the Kremlin-funded channel formerly called Russia Today. “I see absolutely no reason why we should allow information warfare to be carried out on U.K. soil by hostile agents,” he said.
Citing the unusual circumstances, the Metropolitan Police Service put Britain’s counterterrorism police in charge of the case on Tuesday, though it has not yet been determined whether terrorism was involved.
“We’re speaking to witnesses, we’re taking forensic samples at the scene, we’re doing toxicology work,” Mark Rowley, who leads the counterterrorism force, told BBC radio.
Police specialists wearing hazardous-material suits erected a tent over the bench and cordoned off part of the shopping district where the two were found, including an Italian restaurant and a pub, while officers from multiple law enforcement agencies combed the area for evidence.
Responding to reports that some officers had received medical treatment after working at the scene, the police in Wiltshire, the county that includes Salisbury, released a statement on Tuesday confirming that “a small number of emergency services personnel were assessed immediately after the incident, and all but one have been released from hospital.”
But “there does not appear to be any immediate risk to the public,” Kier Pritchard, Wiltshire’s temporary chief constable, said at a brief news conference.
In a town best known for its medieval cathedral and its proximity to the ancient monument of Stonehenge, residents were stunned by their sudden proximity to international intrigue.
“We’re as surprised as anybody at our elevation to the national stage,” said John Walsh, a member of Mr. Skripal’s neighborhood council. He said he had visited residents near Mr. Skripal’s home a few days ago, “sorting out a problem with a hedge — which is more my level of activity, rather than high espionage.”
Ruby Vitorino, a 58-year-old jeweler who came across the crime scene minutes after the police sealed it off, said: “As far as I’m aware, there’s been nothing like this before. It’s a small cathedral town. It’s a small place.”
In 2006, Mr. Skripal, a former colonel in Russian military intelligence, was convicted in a Russian court of being a double agent — while working for Russia, he was secretly passing classified information to British intelligence. In 2010, he was released from prison and sent to Britain as part of a spy exchange with Western agencies.
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He and his wife, Liudmila, who died in 2012, settled in Salisbury, close to several military bases, where he is said to have lectured. Yulia Skripal’s Facebook page says that she graduated in 2008 from the Russian State University for the Humanities in Moscow, lived in several places in southern England beginning in 2010, and by 2016 had moved back to Moscow.
James Puttock, a 47-year-old scaffolder, lives a few doors from Mr. Skripal in Salisbury, on a quiet residential street of semidetached two-story houses, with small lawns outside.
“He was just an ordinary person,” Mr. Puttock said. “I didn’t think he was” a Russian spy.
How do you even know?” he added. “Do I look like a Russian spy?”
In Moscow, the Russian government said it had no information about the apparent poisoning or about Mr. Skripal’s activities in Britain. Dmitri S. Peskov, a Kremlin spokesman, said that Russia had not received any requests for cooperation in the investigation but that it was always open to such requests, the news agency Interfax reported.
Accusations of Kremlin involvement “weren’t long in coming,” Mr. Peskov said. “You know how he ended up in the West, what actions and decisions led him there.”
Mr. Litvinenko, a former K.G.B. officer who was critical of President Vladimir V. Putin, was poisoned with polonium, a radioactive element. The British government was criticized for being slow to blame Moscow or to conduct a thorough investigation, but it did eventually commission an inquiry that concluded that the killing was probably approved by Mr. Putin.
Mr. Rowley, the counterterrorism official, was asked about parallels to the death of Mr. Litvinenko and about reports that a number of people at odds with the Russian government had died in Britain under murky circumstances. He cautioned against jumping to any conclusions.
“There are deaths which attract attention,” Mr. Rowley said. “I think we have to remember that Russian exiles aren’t immortal. They do all die, and there can be a tendency for some conspiracy theories. But likewise, we have to be alive to the fact of state threats, as illustrated by the Litvinenko case.”
On Sunday afternoon, a security camera at a fitness club captured images of a man and a woman, reportedly the Skripals, walking through the shopping area in Salisbury known as the Maltings. The video was time-stamped 3:47 p.m., less than half an hour before the pair were found at 4:15 p.m., incapacitated, on a bench nearby.
The police said that the man and woman had no visible injuries. “At this stage, it is not yet clear if a crime has been committed,” they said in a Twitter post.
The BBC quoted a witness who said that the woman appeared to have passed out, while the man seemed seriously impaired, “doing some strange hand movements, looking up to the sky.” The police said the victims had been unconscious.
When Mr. Skripal was arrested, the Federal Security Service in Russia said that he had started spying for Britain in 1995, when he was stationed overseas, and continued to do so after retiring from the military in 1999.
An earlier version of a picture caption with this article misstated when a Russian intelligence officer and his daughter were found unconscious. It was Sunday, not Monday.