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 detectives had become a homicide investigation with 44-year-old Dawn Sturgess's death at a hospital in Salisbury. She and her boyfriend, Charlie Rowley, 45, were admitted June 30 after falling ill a few miles away in Amesbury; Rowley remains in critical condition.
Tests at Britain's defense research laboratory showed the pair was exposed to Novichok, the same type of nerve agent used to poison a former Russian spy and his daughter in Salisbury in March. Police suspect Rowley and Sturgess handled a discarded item from the first attack, though they have not determined for certain that the two cases are linked.
Britain blames the Russian state for the attack on Sergei Skripal and his 33-year-old daughter -- an allegation Moscow strongly denies.
Prime Minister Theresa May said she was "appalled and shocked" by Sturgess's 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 anti-terrorism police officer, said the death "has only served to strengthen our resolve" to find those responsible.
More than 100 detectives have been working alongside local officers 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 month.
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.
The British defense lab determined earlier that Novichok, a type of nerve agent developed by the Soviet Union during the Cold War, was used on Sergei Skripal, a former Russian intelligence officer once convicted in his homeland of spying for Britain.
The 67-year-old ex-agent was living in Salisbury, a cathedral city 90 miles (145 kilometers) southwest of London, when he was struck down along with his daughter, Yulia, who was visiting him.
They spent weeks in critical condition, but have both been discharged from Salisbury District Hospital, the same hospital where Sturgess died.
The Skripal case, which Metropolitan Police detectives are investigating as attempted murder, sparked a diplomatic crisis between Russia and the West, including the expulsion of hundreds of diplomats from both sides.
Police say the nerve agent that sickened Rowley and Sturgess was the same type that almost killed the Skripals, but scientists haven't been able to tell whether it was from the same batch.
The latest poisonings have further inflamed tensions between London and Moscow. U.K. Home Secretary Sajid Javid has demanded Russia provide information, saying it is unacceptable "for our streets, our parks, our towns to be dumping grounds for poison."
Confirmation from authorities Wednesday that two British citizens were exposed to Novichok shook residents around Salisbury, who thought a months-long cleanup had removed any threat from the powerful nerve agent.
Kier Pritchard, head of the county police force in Wiltshire, acknowledged that Sturgess's death "is likely to raise the level of concern in Amesbury and Salisbury."
But he said health authorities continued to assert the risk to the public was low. Police say they don't think Sturgess and Rowley visited any of the locations decontaminated after the Skripals' poisoning.
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."