LONDON — Tests on two Britons found unconscious in their home determined that they were poisoned by the same nerve agent used against a former Russian spy and his daughter earlier this year, British police said Wednesday.
The discovery triggered new security precautions in the area of Amesbury, England, where the couple were sickened.
The town is seven miles from Salisbury, where former double agent Sergei Skripal and his daughter were poisoned with the military-grade nerve agent Novichok in March. The British government blamed Russia for the attack. Moscow has repeatedly denied any involvement.
Friends and investigators identified the victims as Dawn Sturgess, 44, and Charlie Rowley, 45, who remained in critical condition early Thursday.
Counterterrorism police have taken over the investigation, and authorities acknowledged that the test results would fuel speculation about whether the two poisoning incidents were linked.
"However, I must say that we are not in a position to say whether the nerve agent was from the same batch that the Skripals were exposed to," Neil Basu, assistant commissioner of specialist operations for London Metropolitan Police, said in a statement. "The possibility that these two investigations might be linked is clearly a line of inquiry for us."
Home Secretary Sajid Javid — Britain's interior minister — said in an address to Parliament on Thursday that the incident would "obviously... invoke memories of the reckless murder attempts of Sergei and Yulia Skripal earlier this year."
Javid continued: "The eyes of the world are currently on Russia, not least because of the World Cup. It is now time that the Russian state comes forward and explains exactly what has gone on."
"We do not have a quarrel with the Russian people. Rather it is the actions of the Russian government that continue to undermine our security and that of the international community."
"We will stand up to the actions that threaten our security, and the security of our partners. It is completely unacceptable for our people to be deliberate or accidental targets, or for our streets, our parks, our towns to be dumping grounds for poison," he said.
Sturgess and Rowley were found unconscious at separate times in the same home on Saturday. Investigators initially suspected that they had overdosed on heroin or cocaine, but after observing their symptoms ─ police didn't describe them ─ authorities took samples for lab testing. Those tests came back positive for Novichok, Basu said.
He added that no one else had reported falling ill with similar symptoms.
Investigators are now focusing on how and where Burgess and Rowley came into contact with Novichok, Basu said.
As a "precautionary measure," police cordoned off sites in the Amesbury and Salisbury areas that they believe the two visited just before they were sickened, Basu said.
"I do want to reassure the public, however, that there is no evidence that either the man or woman recently visited any of the sites that were decontaminated following the attempted murders of Sergei and Yulia Skripal," Basu said.
In Salisbury, a supported housing complex for the homeless was cordoned off 840 feet from Zizzi's, the restaurant the Skripals dined at the evening they were poisoned. So, too, was Queen Elizabeth Gardens — a park just 1,300 feet from where the Skripals were found.
Mike Wade, the director of health protection at the southwest branch of the government agency Public Health England, said in a statement Thursday that the risk to the public continued to be low.
Amesbury is about five miles from Stonehenge, the prehistoric monument visited by more than a million people annually.
Francis Whittaker reported from London, and Jon Schuppe from New York.