LONDON >> As the British government scrambled to explain a second nerve-agent poisoning of two people in southwestern England, experts on Thursday were weighing a range of theories about how this could have happened again.

Police said Wednesday that a couple in Amesbury, England, had been sickened by the same class of nerve agent, Novichok, developed by the Russians and used to poison Sergei V. Skripal, a former Russian spy, and his daughter, Yulia Skripal, on March 4 just a few miles away, in Salisbury.

On Thursday, Sajid Javid, the home secretary, went a step further, saying in the House of Commons that government scientists had determined that it was not only “from the Novichok family of nerve agents, but the same type of nerve agent from that family.” But he could not say whether it was from the same laboratory or batch that had produced the poison used in the March attack.

The government blamed Russia for the initial attack, an accusation the Kremlin has denied. The case heightened diplomatic tensions between Russia and the West, leading Britain and its allies to expel more than 150 diplomats, and prompting Moscow to retaliate.

“Many of you will question whether this incident is linked to that one,” Javid said. “That is clearly the main line of inquiry. However, we must not jump to conclusions.”

In an interview on BBC radio, Ben Wallace, the British minister of state for security, said, “The working assumption would be these are victims of the consequences of the previous attack.”

The poisoning of Dawn Sturgess, 44, and her boyfriend, Charlie Rowley, 45, in Amesbury is probably an accidental result of the attack on the Skripals, experts say. Some added that it could be a separate assault, though it was unclear why the couple would have been targeted.

Rowley and Sturgess were being treated at the same hospital that had cared for the Skripals, who survived the attack after weeks in a coma.

Analysts and other experts put forward multiple theories about how the recent poisoning could have occurred.

The nerve agent could have been left on, or in, a discarded object by whoever poisoned the Skripals. It could have been accidentally spread by someone else who had come into contact with it. Or the person who planted it on Sergei Skripal’s front door may have intentionally left it at another location, just to muddy the waters.

“I would put it in the 90 percent-plus likelihood that this was in a discarded item,” like a coat with a container of the nerve agent in a pocket, said Richard Guthrie, coordinating editor of CBW Events, a website on chemical and biological weapons.

Andrew C. Weber, a former U.S. assistant secretary of defense for nuclear, chemical and biological defense programs, said, “It is quite possible that the couple touched a container, applicator or protective gear that was used in the original Russian Novichok attack.”

That hypothesis is plausible, but so are others, said Dan Kaszeta, a former chemical and biological weapons adviser to the White House and the Secret Service. “There’s too many variables here,” he said.

Rowley, who has his own apartment in Amesbury, is a “skip diver,” according to people who knew him, going into big trash-hauling bins to find usable items.

“I think they touched something whilst rummaging and were contaminated like that,” said Kyesha Guest, a friend of the couple. She guessed that “Charlie had been skip diving and touched it, and then touched Dawn.”

Sturgess, who lives in John Baker House, a home in Salisbury for people at risk of homelessness, is a popular member of the community, a mother figure to others in the house, residents said.

“She’s lovely,” said Peter Cook, a retired trucker.

The Amesbury apartment and John Baker House are among six sites that police have cordoned off, saying that the couple may have been contaminated when they were there shortly before taking ill Saturday.

Residents of John Baker House said they were told Thursday morning that they had to leave the building — immediately and permanently — and that other accommodations would be found for them. The street was blocked off by police, and residents said that officers appeared to be searching Sturgess’ room.

Russia categorically denies any involvement in the poisoning of the couple or in the Skripals, said Dmitry Peskov, a spokesman for President Vladimir Putin. “We are, of course, concerned after all by the repeated use of such substances in Europe,” he said in a conference call with reporters. “Although, on the other hand, we do not have information about what kinds of substances were used, how they were used.”

Russian officials also offered alternative theories, including one that says the British could have planted the nerve agent. Sergei Zheleznyak, deputy speaker of Parliament, suggested that British authorities had concocted the case to sully Russia’s image while the country was hosting the World Cup.

“A huge number of British fans, despite the warnings from their government, came to support their team,” Zheleznyak told state television. “Their impressions are just destroying everything British propaganda built over the past few years. To break up this flow of really positive emotions that the British fans are sharing, they had to put something like this in the mass media.”

Nikolai Kovalyov, a former director of the Federal Security Service, the successor to the KGB, suggested a rogue scientist from the Porton Down laboratory, near Salisbury, was conducting experiments on people living nearby.

But Javid said, “We can anticipate further disinformation from the Kremlin, as we saw following the Salisbury attack.”

On social media, critics of the British government questioned how the nerve agent could have survived four months without breaking down into less-harmful components, as many other toxins would. But experts had an explanation.

“The stuff is relatively persistent — it’s designed to last a long time,” Kaszeta said.

That would be especially true if the toxin were suspended in a gel, he said, adding that it was a strong theory among investigators looking into the March attack. A thinner liquid would be more easily broken down if it came into contact with water, but a gel would repel water.

Weber agreed that Novichok had been engineered to resist decay, adding, “I believe it was mixed with some type of gel to help it adhere and persist.”

Police have made clear in the Skripal case that “this stuff is very persistent in the environment,” Guthrie said. In addition, he said, a container — such as a syringe wrapped in a plastic bag or a screw-top plastic jar — would have protected the agent from water and decay.

Speaking in Parliament, Javid said he had been advised that the time needed for this particular agent to deteriorate “can be months and months.”

As for how much of the substance the Skripals’ assailant had, “that’s anybody’s guess,” Kaszeta said. “Were the guys who did this given more than they needed?” he asked. “The container might even have fingerprints on it. It’s a smoking gun, but killers sometimes even leave a smoking gun at the scene of a crime.”

Novichok can be absorbed through the skin, experts say, but it could take hours for the victim to become ill. Police and health officials have asked those who went recently to one of the cordoned-off sites to wash their clothes, and to use disinfectant or wet wipes to clean personal items like cellphones or jewelry.

Sally Davies, chief medical officer of England, said anyone in the area should refrain from picking up unknown objects from the ground.

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