"The GRU is a highly disciplined organisation with a well-established chain of command", she said.
They returned the following day, when they are believed to have smeared Novichok, a military-grade nerve agent developed by the Soviet Union during the Cold War, on Skripal's front door.
The attack on the Skripals sparked a Cold war-style diplomatic crisis between Russia and the West. Britain, the U.S. and other countries expelled scores of Russian diplomats while Moscow has retaliated in kind.
A spokeswoman for the Russian foreign ministry said Wednesday it had no knowledge of Petrov or Boshirov, and accused British authorities of manipulating information in the case.
Mr Basu said it is likely the suspects, who are aged around 40, were travelling under aliases and Petrov and Boshirov are not their real names.
Britain's interior minister says Moscow must clarify its use of the poison, drawing a Russian rebuke that London was playing "dirty political games" and must apologise.
Mr Basu said: "We have no evidence that they re-entered the United Kingdom after that date". The result, he said, was "a ideal cover for smuggling the weapon into the country, and a flawless delivery method for the attack against the Skripal's front door".
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From there, they travelled to Salisbury on 4 March where Mr Skripal's front door was contaminated with Novichok. Britain has issued a European Arrest Warrant for the suspects, meaning they can be detained if they leave Russian Federation for another European country, but assistant police commissioner Neil Basu conceded it was "very very unlikely" police would be in a position to arrest them any time soon. Her boyfriend Charlie Rowley also became seriously ill during that incident, and police officer Nicholas Bailey was hospitalised by exposure to the Skripals at the time of the first incident.
Prosecutors will not be applying to Russian Federation for the extradition of the two men, as no agreement exists between the countries, but a European Arrest Warrant has been obtained in case the pair are spotted outside of Russian Federation.
Novichok poisoning victim Charlie Rowley says news that two suspects have been identified in the Salisbury attack is progress, and he wants to see the men brought to justice. They stayed in a hotel in London - where traces of Novichok were also found - and made two trips to Salisbury.
Wiltshire local woman Dawn Sturgess died after inadvertently spraying herself with the Novichok virus which was inside a discarded perfume bottle.
The two men are officers in Russia's military intelligence agency, the GRU, the Independent reported. He said they arrived in Britain on March 2 and left on March 4.
British authorities and the global chemical weapons watchdog say the victims were exposed to Novichok, a type of military-grade nerve agent developed by the Soviet Union during the Cold War.
On September 4, the OPCW said laboratory tests showed that Sturgess's death was caused by the same substance that poisoned Skripal and his daughter.
The case has been likened by British politicians to the murder of Russian dissident and former KGB spy Alexander Litvinenko, who was poisoned with a rare radioactive isotope in a London hotel in 2006. Later, she told the state news agency Interfax that the ministry demanded that the British supply Russian Federation with the fingerprints of the two men accused of the attempted murder of the Skripals.
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