A woman who was exposed to the nerve agent Novichok in Wiltshire county in South West England died in hospital on Sunday, The Guardian reported.

The police launched a murder investigation after Dawn Sturgess from Durrington, died after handling an item contaminated by the nerve agent on June 30. Her partner, Charlie Rowley, who also fell ill after being exposed to Novichok, remains in critical condition in a hospital.

The couple were found unconscious at a house in Amesbury town of Wiltshire. Amesbury is about 12.87 km from Salisbury, the place where former Russian spy Sergei Skripal and his daughter were poisoned by the same gas in March. Investigators are still trying to determine how the couple were exposed to the nerve agent.

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 tweeted on Monday.

Head of Britain’s counter-terrorism policing, Assistant Commissioner Neil Basu, said it was “shocking and tragic news”. “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, according to AFP.