A couple poisoned by a Russian nerve agent in Salisbury "must have handled" a contaminated container, Britain's top counter-terrorism officer has said.
Dawn Sturgess, 44, died in hospital on Sunday after being accidentally exposed to novichok on 29 June. Charlie Rowley, 45, remains in a critical condition.
Security services believe the couple were inadvertently exposed to the same nerve agent used to attack Sergei Skripal and his daughter four months ago.
Assistant commissioner of specialist operations Neil Basu, who is leading the investigation, said: "Our focus and priority at this time is to identify and locate any container that we believe may be the source of the contamination.
"In the four months since the Skripals and Nick Bailey were poisoned, no other people besides Dawn and Charlie have presented with symptoms.
"But their reaction was so severe, it resulted in Dawn’s death and Charlie being critically ill. This means they must have got a high dose, and our hypothesis is that they must have handled a container that we are now seeking."
The government has accused Russia of using British streets, parks and towns as “dumping grounds for poison” after the incident, which provoked fresh public health fears following assurances Salisbury had been safely decontaminated.
Theresa May said she was “appalled and shocked” by Ms Sturgess' death and sent her thoughts and condolences to the family.
Mr Basu said detectives were unable to say for sure if the novichok the pair were exposed to was the same that struck down former Russian double agent Sergei Skipal and his daughter Yulia, but it remained their main line of inquiry.
He said police had identified a red Ford transit van Mr Rowley had travelled in on 30 June, just hours before being rushed to Salisbury District Hospital.
The van was taken by military personnel to the nearby defence laboratory at Porton Down, where scientists will carry out tests on the vehicle.
Three other men were also in the van, Mr Basu added, though none were unwell or had shown signs of being exposed to a nerve agent.
Praising the "tremendous stoicism" of people of Wiltshire, he told journalists outside Scotland Yard that 21 people have raised concerns about being exposed but have been given the all clear.
The government's Cobra emergencies committee was due to meet at 1pm on Monday to discuss the case.
News of Ms Sturgess' death came hours after the home secretary announced that the government had "no current plans" for additional sanctions on Russia, following a spate of diplomatic expulsions following the attack on Mr Skripal in March.
Sajid Javid later expressed his condolences and added: "Police and security officials are working around the clock to establish the full facts.
“This desperately sad news only strengthens our resolve to find out exactly what has happened.
“As I said earlier when I visited Amesbury and Salisbury, the government will continue to provide the local community all the support it needs.”
The government attributed the attack on Mr Skripal to Russia because of its history of state-sponsored assassinations abroad, the Soviet development of novichok and the Kremlin's refusal to explain how it might have lost control of the nerve agent.
Russia has denied any involvement in either incident and the Kremlin previously expressed hope that Ms Sturgess and Mr Rowley would recover.
