March 28 (UPI) -- The Russian double agent and his daughter who were attacked with a rare poison may have came in contact with the substance at their front door, British officials said Wednesday.
On March 4, Sergei Skripal, 66, and Yulia Skripal, 33, were found in critical condition at an outdoor shopping center in Salisbury. They remain in critical condition and the cause of their poisoning is till under investigation.
British law enforcement officials said new evidence will have them focus their investigation in and around the Skripal home.
"At this point in our investigation, we believe the Skripals first came into contact with the nerve agent from their front door," said Scotland Yard Deputy Assistant Commissioner Dean Haydon.
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Haydon said people living in Skripal's neighborhood will see investigating officers in the coming days, but "the risk remains low and our searches are precautionary."
Scotland Yard described its investigation as "one of the largest and most complex investigations undertaken by British counter-terrorism policing."
Approximately 250 counter-terrorism detectives are working the case and about 500 witnesses have been identified and questioned.
British officials have blamed the Russian government for the attack based on evidence the poison is possibly linked to a Russian government chemical weapons lab.
Russia has denied any involvement in the attack.
Sergei Skripal is a former Russian military intelligence official who was convicted in 2006 of passing classified information to the British government. In 2010, he was pardoned by then-President Dmirtry Medvedev before being included in a group of four prisoners released to the United States in exchange for 10 Russian spies arrested by the FBI.
After the exchange, Skripal was sent to Britain.
