British police rushed to solve a murder mystery on Monday after a woman died following exposure to the nerve agent Novichok, four months after the same toxin was used against a former Russian spy in an attack that Britain blamed on Moscow.
Prime Minister Theresa May said she was "appalled and shocked" by the death of Dawn Sturgess, a 44-year-old mother of three who had been living in a homeless hostel in the city of Salisbury in southwest England.
Sturgess and Charlie Rowley, 45, fell ill last weekend in the town of Amesbury, near Salisbury, the city where former double agent Sergei Skripal and his daughter Yulia were attacked with the Novichok nerve agent in March and have since recovered.
Local MP John Glen said the local community was "anxious" after police opened a murder inquiry, although health officials have said the danger to the general public is low.
Glen told BBC radio the two may have handled a contaminated object because of their "habit of looking into bins" and police were trying to work out "how they came into contact with this nerve agent and when".
Britain and its allies accused Russia of trying to kill the Skripals, prompting angry denials and sparking an international diplomatic crisis.
Police said they would be led by the evidence but confirmed a link between the Amesbury case and the Salisbury attack was a main line of inquiry.
Hospital staff 'worked tirelessly' 
Interior minister Sajid Javid last week demanded answers from Moscow, saying he would not accept Britain becoming a "dumping ground for poison".
Russia hit back, denouncing Britain for playing "dirty political games".
Police said the British couple were believed to have become exposed to Novichok by handling a "contaminated item", with speculation that it could have been the container used to administer the nerve agent to the Skripals.
However, police and public health officials insist the risk to the wider public remains low.
A police officer was tested for possible exposure to the deadly nerve agent over the weekend but was given the all-clear.
Christine Blanshard, medical director at Salisbury District Hospital, where Sturgess and Rowley were being treated and where the Skripals were hospitalised, told the Daily Telegraph that staff had "worked tirelessly to save Dawn".
"Our staff are talented, dedicated and professional and I know today they will be hurting today," she said.
'Reckless and barbaric act' 
The prime minister said: "Police and security officials are working urgently to establish the facts of this incident, which is now being investigated as a murder."
Assistant Commissioner Neil Basu, the head of Britain's counter-terror police, said Sturgess's death "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".
He added that the other victim "remains critically ill in hospital and our thoughts are with him and his family as well".
Residents of the homeless hostel in Salisbury where Sturgess lived, which was evacuated after the couple fell ill, expressed their devastation at the news of her death.
"It could easily have happened to anyone, to me or my partner," 27-year-old Ben Jordan told AFP late on Sunday. "We are really, really sad. I am praying for Charlie."
Around 100 counter-terrorism officers are helping in the investigation, which police said Friday could take "weeks and months".
So far, there is no evidence that the couple visited any of the sites involved in the Skripal case.
"Detectives will continue with their painstaking and meticulous work to gather all the available evidence so that we can understand how two citizens came to be exposed with such a deadly substance that tragically cost Dawn her life," Basu said.
Sturgess collapsed on the morning of June 30 and was taken to hospital. That afternoon, Rowley fell ill at the same address in Amesbury and was also hospitalised.
It was not until Wednesday, however, that the government's Defence Science and Technology Laboratory at Porton Down confirmed their exposure to Novichok.
"Detectives are working as quickly and as diligently as possible to identify the source of the contamination, but this has not been established at this time," police said on Sunday.
"We are not in a position to say whether the nerve agent was from the same batch that the Skripals were exposed to."
The Skripals were released from hospital but the investigation into the attack on them continues. No arrests have been made.