LONDON — A 44-year-old British woman who was exposed to a nerve agent died on Sunday, bringing new urgency to a four-month-old diplomatic standoff in which Britain has accused Russia of sending the poison to a small city in southern England in a botched attempt to kill a former spy.
The British authorities have now opened a murder investigation.
“Today is the day we hoped would never come,” said Chief Constable Kier Pritchard of the Wiltshire Police.
The police say the woman, Dawn Sturgess, 44, was most likely exposed accidentally to residue from a Soviet-developed, military-grade nerve agent used in a March attack on the former spy, Sergei V. Skripal. He lived near Ms. Sturgess in Salisbury.
After Mr. Skripal and his daughter, Yulia, collapsed, British officials declared confidently that Russia was at fault, in large part because of the obscure poison involved. It was one of a strain of nerve agents known as Novichok that they say is kept under tight control by the Russian authorities.
Russia has denied any involvement.
In recent months, investigators have said little about the evidence they have gathered and they have named no suspects. And the Skripals recovered, allowing the crime to fall off the front pages.
Ms. Sturgess’s death is likely to change that, forcing Britain to reassert its suspicions just as Russia is enjoying an international spotlight as host of the 2018 World Cup.
“It becomes a murder, and it’s involving a British national rather than a Russian national,” said James Nixey, head of the Russia and Eurasia program at Chatham House, a research group in London. “It stiffens resolve where resolve was ebbing.”
It also presents another urgent challenge for British authorities: The contamination, it appears, was more widespread than initially thought.
“It was widely assumed that, from the point of view of exposure, it was over,” Mr. Nixey said. “If there was still stuff out there in concealed containers, then I think they will be unusually worried. It is, after all, the first duty of the state to protect its citizens.”
Ms. Sturgess and her boyfriend, Charlie Rowley, 45, appear to have been accidental victims. Each had struggled with addiction, and when they fell ill on July 1, the police initially suspected an overdose or contaminated batch of drugs.
But their symptoms — foaming at the mouth, pinpoint pupils and hallucinations — were similar to those that had emerged with the three previous victims: the Skripals, and a detective who was exposed while responding to the crime.
Ms. Sturgess and Mr. Rowley were physically fragile after years of substance abuse, which compromises the liver’s ability to detoxify the body. Mr. Rowley remains in critical condition.
The police have said that the nerve agent that poisoned them on Saturday was the same type used on Mr. Skripal and his daughter in March, but could not confirm that it was the same strain or batch.
Neil Basu, a top counterterrorism official with the Metropolitan Police, said the authorities were continuing to look into possible connections with the poisoning of the Skripals.
“This terrible news has only served to strengthen our resolve to identify and bring to justice the person or persons responsible for what I can only describe as an outrageous, reckless and barbaric act,” Mr. Basu said in a statement on Sunday evening. “Detectives will continue with their painstaking and meticulous work to gather all the available evidence so that we can understand how two citizens came to be exposed with such a deadly substance that tragically cost Dawn her life.”
Ms. Sturgess’s son, Ewan Hope, 19, told The Mirror, a daily newspaper, that he had visited her in the hospital, but was allowed to touch her only while wearing gloves.
“I touched her hair through the gloves and told her: ‘I love you, Mum. I just want you to get better,’” he told the newspaper before she died. “I’m worried I’m going to lose my mum.”
In Facebook posts, Ms. Sturgess had written of her difficulty finding housing, but recently she settled into a room at John Baker House, a supported-living facility in Salisbury for people with drug and alcohol problems. Her posts brightened in February 2017 when she began her relationship with Mr. Rowley, a recovering heroin addict who rented an apartment in Amesbury, eight miles away.
“Fell in love … never bodes well for me,” she wrote. “I trust Charlie with my life and he gets me the best gifts ever.”
The police have meticulously traced the route that Ms. Sturgess and Mr. Rowley took during the hours before they collapsed, in an effort to find an object — perhaps an ampul or syringe — that the two had handled.
It is not yet clear how the British public will respond to Ms. Sturgess’s death, said Olga Oliker, director of the Russia and Eurasia program at the Center for Strategic and International Studies, a research group in Washington.
“It’s a little bit like the election interference in the U.S. elections — the interference happens, and when new evidence shows up, the anger rises,” she said. “In this case you have the death of a person, which makes it different.”
Ms. Oliker called the attack on the Skripals a “strange case,” because it appeared to have been carried out carelessly.
“Past cases that looked similar were certainly much more professionally carried out,” she said. “This is sloppy — so much evidence left behind, material left behind where people could find it.”
After the new poisonings became public, Russia denied any involvement, and officials suggested alternative explanations. A Russian lawmaker said the British authorities might have concocted the case to sully Russia’s image while the country was hosting the World Cup soccer tournament.
Though there have been a string of suspicious deaths of anti-Kremlin figures in Britain in recent years, only one has culminated in a full investigation and extradition request. The victim was Alexander V. Litvinenko, a former Federal Security Service officer who was killed in 2006. He was served tea spiked with Polonium 210, a radioactive isotope.