The killing of Dawn Sturgess, a mother of three, will increase pressure on President Putin after Sajid Javid, the home secretary, accused the Kremlin of using Britain as a dumping ground for poison.
Britain has accused Russian Federation of poisoning the Skripals with Novichok in what is the first known offensive use of such a chemical weapon on European soil since World War Two.
Fire trucks and emergency response vechiles are parked outside a residential house in Amesbury, southern England, July 7, 2018, where a couple was exposed to the same nerve agent as the Russian double agent Sergei Skripal.
Police say the nerve agent that sickened Rowley and Sturgess was the same type that nearly killed the Skripals, but scientists haven't been able to tell whether it was from the same batch.
They were believed to have become exposed to Novichok by handling a container, and a link to the Salisbury attack in March is a main line of investigation. Britain blames Russian Federation for the attack, however, Russian Federation has denied the allegation.
"Dawn's family has asked the media to kindly respect their privacy at this hard time".
"Police and security officials are working urgently to establish the facts of this incident, which is now being treated as murder", May said.
She said: "The government is committed to providing full support to the local community as it deals with this tragedy".
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Mr Rowley, 45, from Amesbury, remains in a critical condition in hospital.
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.
UK Prime Minister Theresa May said Sunday she was "appalled and shocked" by the death of Sturgess, according to a statement released by 10 Downing Street.
The hospital's medical director, Christine Blanshard, told the BBC that hospital staff worked tirelessly to save Sturgess.
Ms Sturgess's family are being given support by specialist officers and have asked for privacy.
Authorities later determined that both people had been exposed to Novichok, a military-grade nerve agent produced by the Soviet Union decades ago.
Public Health England said the risk to the general public "remains low".
Yulia Skripal, Sergei's daughter, was in a coma for 20 days after she was attacked and was eventually discharged about five weeks after the poisoning.
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