British police say a woman who fell critically ill after being exposed to the same nerve agent used in the poisoning of Russian ex-spy Sergei Skripal and his daughter has died.
Britain's Metropolitan Police said in a statement that Dawn Sturgess, a 44-year-old mother of three, died in the hospital on July 8 after being exposed to the nerve agent Novichok.
The death is now being investigated as a murder, police said in a statement.
Prime Minister Theresa May said she was "appalled and shocked" by the death of Sturgess.
"Police and security officials are working urgently to establish the facts of this incident, which is now being investigated as a murder," she wrote on Twitter.
Neil Basu, the head of U.K. counterterrorism policing, called it "shocking and tragic news."
"Dawn leaves behind her family, including three children, and our thoughts and prayers are with them at this extremely difficult time," he added.
"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," Basu said.
British authorities say Sturgess and her boyfriend, 45-year-old Charlie Rowley, were found unconscious and in critical condition at a house in Amesbury on June 30, less than 20 kilometers from Salisbury, where Skripal and his daughter, Yulia, were poisoned with the nerve agent in March.
Rowley remains hospitalized in critical condition.
Police continue to investigate how they may have been exposed to the toxin.
The incident involving the Skripals, who both recovered from their exposure to Novichok after weeks in a hospital, triggered a diplomatic crisis between Moscow and the West.
Britain blamed Russian President Vladimir Putin's government for the attack with a military-grade chemical weapon that was developed in the Soviet Union. Russia rejects the accusation.
Britain expelled 23 Russian diplomats it says were spies and suspended high-level contacts over the incident.
Russia ordered the expulsion of 23 British diplomats and the closure of Russian operations of the British Council in Russia, which promotes cultural ties between the two countries, in retaliation.