LONDON — A woman who was poisoned by a military-grade nerve agent in southwest England died Sunday, eight days after police think she touched a contaminated item that has not been found.
London’s Metropolitan Police force said the case had become a homicide investigation now that 44-year-old Dawn Sturgess had died in a hospital in Salisbury. She and her boyfriend, Charlie Rowley, 45, were admitted June 30. Rowley remains in critical condition.
Police said tests showed the pair was exposed to Novichok, the same type of nerve agent used to poison former Russian spy Sergei Skripal and his daughter, Yulia, in Salisbury in March. Police suspect Rowley and Sturgess handled an item from the first attack, which Britain blames on Russia.
Moscow denies involvement.
Prime Minister Theresa May said she was ‘‘appalled and shocked’’ by Sturgess’ death.
‘‘Police and security officials are working urgently to establish the facts of this incident, which is now being treated as murder,’’ May said.
Assistant Commissioner Neil Basu, Britain’s top antiterrorism police officer, said the death ‘‘has only served to strengthen our resolve’’ to find those responsible.
More than 100 police officers have been working to locate a small vial or other container thought to have held the nerve agent that sickened the two. Officials say the search and cleanup operation will take weeks or even months.
Counterterrorism police are also studying roughly 1,300 hours of closed-circuit television footage in hopes of finding clues about the couple’s activities in the hours before they became violently ill.
Detectives want to know where the couple was to get new leads on where the contamination might have occurred.
Britain maintains the March attack on the Skripals had been ordered by the Russian government, a charge denied by representatives of President Vladimir Putin of Russia.
The case led to the expulsion of Russian diplomats from Britain, the United States, and other countries, and tit-for-tat retaliation by Moscow.
The new poisoning has frightened some residents who thought an extensive cleanup had removed the threat of any further Novichok exposure.
Hospital officials said late Saturday that a number of people including a police officer had sought medical advice in the last week but had been found not to need any treatment.
John Glen, the Conservative Party legislator for the region, said the new poisoning has threatened an economic rebound from the slowdown caused by the attack on the Skripals.
‘‘We need to establish quickly what they came into contact with and where,’’ he said. ‘‘The sentiment in the city is frustration; we want to get back to normal.’’
Before the death was announced, Home Secretary Sajid Javid visited Salisbury and nearby Amesbury, where the couple fell ill, on Sunday to reassure residents that the risk to the public remains low.
Javid said the area is open for business and urged people to visit what he called one of the most beautiful parts of the country.
Javid was the first senior official to visit the area since Sturgess and Rowley were hospitalized in critical condition last week.