Police say they believe the nerve agent used to poison former Russian spy Sergei Skripal and his daughter was smuggled to Britain in a counterfeit Nina Ricci perfume bottle and applied to the front door of Skripal's house.
Sergei Skripal, a former colonel in Russian military intelligence who exposed dozens of agents to Britain's MI6 foreign spy service, and his daughter, Yulia, were found unconscious on a public bench in Salisbury on March 4.
Britain and its allies blame Russian Federation for the attack, a claim Moscow vehemently denies.
British Prime Minister Theresa May described the suspects as Russian military intelligence officers and alleged the March attack in Salisbury was approved by senior government officials in Moscow.
Britain, France, Germany, Canada and the United States pledged on Thursday to work to disrupt "the hostile activities of foreign intelligence networks" and called on Russian Federation to disclose its nerve agent programme. She said the worldwide community must take steps to safeguard people against the use of chemical weapons and "the threat of hostile foreign interference".
A photo made available by the London Metropolitan Police showing both suspects at Salisbury train station on March 4.
Pierce said "they are progressing well".
Ben Wallace, the UK's security minister, said that Mr Putin and his government directed, controlled and funded the GRU.
Dawn Sturgess, 44, became an indirect casualty of the poisoning and she died after she touched the poisoned item with her hand.
British authorities said that a European arrest warrant had been issued for the two Russians, identified as Aleksandr Petrov and Ruslan Boshirov, although it is suspected the names are aliases.
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He said of the "sickening and despicable" attack: "Unequivocally, crystal clear this was the act of the Russian state, two Russian nationals sent to Britain with the sole goal of carrying out a reckless assassination attempt".
Mr Putin's spokesman Dmitry Peskov said that Mrs May's accusations are "unacceptable" and that "no-one in the Russian leadership" has anything to do with the poisoning, while foreign ministry spokeswoman Maria Zakharova accused the United Kingdom and USA of a "witch hunt" against Russia.
"Ultimately of course he is responsible, he is leader of the state", he said.
In a speech in October a year ago, MI5 director general Andrew Parker referenced the agency's work "against espionage and other clandestine activity by Russian Federation and other foreign states who seek to do Britain harm".
But he warned: "Its ubiquity brings anonymity to terrorists, paedophiles and cybercrime gangs who law enforcement and intelligence agencies are trying to stop".
Sue Hemming of the Crown Prosecution Service said the United Kingdom wouldn't ask Moscow to hand the men over because Russian law forbids extradition of its citizens.
On Thursday, she accused Britain of "concealing the evidence" and demanded that it share the suspects' fingerprints and other data. The poisoning ignited a diplomatic confrontation in which hundreds of envoys were expelled by both Russian Federation and Western nations. But there is limited appetite among Britain's European allies for further sanctions against Moscow.
Sergei V. Skripal was a little fish. "We have lost all hope of finding the truth of who is guilty", he said. Her partner, Charlie Rowley, was also stricken but survived.
This story was reported by The Associated Press.