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 nearly killed 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 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 Novichok in March. They have since recovered.
The British government has called a meeting of its COBRA emergencies committee for 1:00pm (1200 GMT).
The Kremlin said it would be "absurd" to suggest Russia was involved in the death of Sturgess.
"We don't know that Russia has been mentioned or associated with this," President Vladimir Putin's spokesman Dmitry Peskov said.
"We consider that in any case it would be quite absurd."
Russia is "deeply concerned by the continuing appearance of these poisonous substances on British territory," which "present a danger not just for the British but for all Europeans," Peskov added.
Britain and its allies accused Russia of trying to kill the Skripals, prompting angry denials and sparking an international diplomatic crisis.
- 'No guarantees' -
Police said they could not yet say whether the nerve agent in the Amesbury case was linked to the Salisbury attack -- but it was their main line of inquiry.
The head of Britain's counter-terror police also said he could not rule out further contaminations.
"I simply cannot offer any guarantees," said Assistant Commissioner Neil Basu, who is leading the investigation.
He said people in Salisbury should not pick up strange items such as needles, syringes or unusual containers.
Whilst 21 other people have come forward with health concerns, they have been screened and "all been given the all-clear", he said.
Police and public health officials insist the risk to the wider public remains low.
Police said that given the deadly dose, 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.
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 newspaper that staff had "worked tirelessly to save Dawn".
"This latest, horrendous turn of events has only served to strengthen the resolve of our investigation team as we work to identify those responsible for this outrageous, reckless and barbaric act," said Basu.
He said Rowley remained critically ill in hospital.
- 'Praying for Charlie' -
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. "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.
On Wednesday, the government's Defence Science and Technology Laboratory at Porton Down confirmed their exposure to Novichok.
Police said Sunday they were not yet able to say whether the nerve agent was from the same batch that the Skripals were exposed to.
The Skripals have been released from hospital but the investigation into the attack on them continues. No arrests have been made.