Authorities in the UK have opened a murder investigation after a British woman died from exposure to a Soviet-era nerve agent that had previously sickened a former Russian spy and his daughter.

Prime Minister Theresa May said that authorities were “working urgently to establish the facts of this incident,” adding that she was “shocked and appalled” by the death of 44-year-old Dawn Sturgess on Sunday evening.

Speaking at Scotland Yard on Monday, Neil Basu, the assistant commissioner for counterterrorism with London’s Metropolitan Police, said there was no evidence that the nerve agent that caused Sturgess’ death was linked to the attack on the Sergei and Yulia Skripal, but that this remained the “main line of enquiry.”

UK Home Secretary Sajid Javid last week called on Russia to provide an explanation for the incident after it was confirmed that Sturgess and another person, 45-year-old Charles Rowley, were exposed to Novichok, the same substance that sickened the Skripals in March.

Sturgess and Rowley fell ill in the town of Amesbury in Wiltshire, about eight miles north of Salisbury, where the Skripals were poisoned. Yulia Skripal was discharged from the hospital in April and her father was released a few weeks later.

Kremlin spokesman Dmitry Peskov dismissed any Russian involvement in the Amesbury poisoning, calling it “more of a UK problem.”

Asked by CNN to comment on remarks by a Russian official who suggested that the poisoning had been “done on purpose” to derail the World Cup and ongoing summit preparations,” Peskov said it was “absurd” to mention Russia in connection with the incident, but added, “the nature of the incident raises concerns and poses a big threat.”

Police believe Sturgess and Crowley were exposed to the nerve agent Novichok after handling a contaminated item. Both were admitted to Salisbury District Hospital nine days ago after falling ill, the same hospital where the Skripals were treated.

“Our focus and priority at this time is to identify and locate any container that we believe may be the source of the contamination,” said assistant commissioner Basu Monday.

“Their reaction was so severe, it resulted in Dawn’s death and Charlie being critically ill. This means that they must have got a high dose and our hypothesis is that they must have handled a container we are now seeking.”

There is no evidence that either Sturgess or Rowley visited any of the sites that were decontaminated following the attack in March, according to authorities, who say they are continuing to build a time line of the couple’s movements.

Neil Basu said that, while he cannot “offer guarantees,” the “risk to the general public at this time remains low.”

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