Describing the difficulties of the investigation, Metropolitan Police Assistant Commissioner Neil Basu said: 'There are multiple scenes, there are only limited numbers of scientists and forensic medical officers - all of whom are volunteers by the way - to do what is a really, really unsafe piece of work for obvious reasons.

Sturgess and Rowley fell ill four months after novichok was used to try and assassinate former Russian spy Sergei Skripal and his daughter Yulia, after it was smeared on the door of their Salisbury home.

Police said on Wednesday they had spoken to the man poisoned with a nerve agent as detectives sought to discover how he and his partner, who died on Sunday, were exposed to Novichok. Moscow rejected the accusations and hit back by expelling Western diplomats. Sturgess died in hospital on July 8 while Rowly remains in critical condition.

It comes a day after the hospital said he had regained consciousness.

Novichok is the name for a group of Russian chemical weapons designed by the Soviet Union in the 1990s, and are among the most toxic substances ever created. Sturgess and Rowley were found at a house in Amesbury, 11 kilometers (7 miles) away.

Basu has previously said that due to the high dose that Sturgess and Rowley received, the hypothesis was they must have handled a contaminated container.

"Dawn's death has been devastating for us", they said in a statement on Tuesday. "Our focus and priority at this time is to identify and locate any container that we believe may be the source of the contamination".

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He also wanted an easier sentence for Dwight and Steven Hammond, who had both previously rejected his assistance. When confronted by a firefighter the following day, Hammond admitted to starting the fire, Williams wrote.

Britain and its allies have blamed Russia for the attack on the Skripals, prompting the biggest Western expulsion of Russian diplomats since the Cold War.

Searches have taken place in Amesbury, where the pair fell ill, and in Salisbury where Ms Sturgess lived.

Police initially thought they had consumed a contaminated batch of illegal drugs but their symptoms prompted further testing which confirmed their exposure to Novichok. Ms Sturgess has since died in hospital.

Yulia Skripal, Sergei's daughter, was in a coma for 20 days after she was attacked and was eventually discharged about five weeks after the poisoning.

"Our staff will continue to work hard to provide the care that Charlie needs", she said.

Lorna Wilkinson, the hospital's director of nursing, said "his condition is now serious, but stable".

Basu, head of Scotland Yard's counter-terror division, said it was "implausible" to rule out any link between the two nerve agent incidents given their proximity.