British police have launched a murder inquiry after a woman exposed to the nerve agent Novichok in Wiltshire, England, died. Cops released a statement saying the woman—named as 44-year-old Dawn Sturgess—died Sunday evening. Sturgess fell critically ill on June 30 alongside her partner Charlie Rowley, 45, who remains in critical condition in a hospital. Police said specialist counterterror police were leading the murder investigation and that about 100 detectives were searching for the culprit. Assistant Commissioner Neil Basu, the head of U.K. Counter Terrorism policing, said: “This is 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.” British Prime Minister Theresa May said she was “appalled and shocked” by the death, which comes four months after she blamed Russia for the poisoning of Sergei Skripal and his daughter Yulia with a Novichok nerve agent in nearby Salisbury. That incident launched a round of expulsions of Russian diplomats by governments around the world.