"This is shocking and tragic news", Neil Basu, of the UK's counterterrorism agency, said. Rowley remains in critical condition and in a coma at the Salisbury hospital.
The attack on the Skripals worsened the already tense relations between Russia and the West, prompting the expulsion of Russian diplomats and embassy workers from several countries and retaliatory expulsions by Moscow.
Scotland Yard has begun a murder investigation after Ms Sturgess died in hospital yesterday evening.
A post-mortem examination of Ms Sturgess, from Durrington, is due to take place and her family has been informed, police said.
Despite the lack of evidence and deepening mystery many have been quick to point the finger at Russian Federation and brand the Amesbury poisoning "collateral damage".
"What I am very confident of, with the amount of police work that's going on here, the support both from Wiltshire Police but also more than 100 officers from the counter-terrorism network involved as well, that they will eventually, one way or the other, get to the bottom of this", he said.
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Russian ex-agent Sergey Skripal, who received asylum in United Kingdom after being released in a spy Exchange in 2010, and his daughter Yulia were treated for weeks by poisoning with a nervous agent, before receiving medical discharge.
Last month The Times reported Sergei Skripal's home and a house belonging to the police officer Detective Sergeant Nick Bailey poisoned by a nerve agent in Salisbury was to be bought by the British government in a £1m deal that also includes cars and other family possessions.
Mr Javid said after visiting Wiltshire towns on Sunday: "This desperately sad news only strengthens our resolve to find out exactly what has happened".
Assistant Commissioner Basu said the couple must have received a "high dose" of Novichok when they handled a vessel containing the nerve agent.
Rowley regularly scavenged for goods to fix and sell, leading to the possibility that he and Sturgess might have inadvertently found the container used to transport the Novichok. "We believe that in any case this would be quite absurd", Kremlin spokesperson Dmitry Peskov said.
Police are yet to recover that item but Public Health England said the risk to the public is low and warned against picking up "any odd items such as needles, syringes or unusual containers". Officials said they had come into contact with the deadly nerve agent Novichok, which was manufactured in the Soviet Union during the Cold War. "Their evidence is vital for both public safety and for the investigation".