Former double agent Sergei Skripal, 66, and his daughter Yulia, 33, have been in hospital in a critical condition since Sunday when they were found unconscious on a bench in the southern English cathedral city of Salisbury.
She said: "This investigation is focused on making sure that we keep people safe and also that we collect all the evidence so that when it comes to attribution (of the attack) we will be absolutely clear where it should be", she said.
He said: "There obviously are some indications the officer, and I'm very sorry that he has been injured, has actually been to the house, whereas there was a doctor who looked after the patients in the open who hasn't been affected at all".
Its statement continued "over time, repeated skin contact with contaminated items may pose a small risk to health".
Yesterday, police said they have identified more than 200 witnesses and 240 pieces of evidence.
"This is precautionary advice aimed at only those people who were at the venues at these times, which I believe to be below 500 people". Litvinenko's death sparked a major crisis in British-Russian relations, as many public figures in the West accused the Russian government of alleged involvement.
Some British lawmakers have asked for a high-level investigation of a string of serious mishaps involving former Russia spies and foes of Russian President Vladimir Putin who have taken up residence in Britain. Officials haven't said what type of nerve agent was used.
At an ambulance station a short distance from the city centre, troops in light grey overalls, purple gloves and gas masks covered ambulances with black tarpaulins as they prepared to remove them.
"We have now learned there has been some trace contamination by the nerve agent in both the Mill Pub and Zizzi restaurant in Salisbury", Davies said.
Chapman is a mother of one, giving birth to a son in summer 2015.
Petrova described Sergei Skripal as "a good, kind man" and said it was terrible that the "whole family disappeared in basically one instant".
After the first travel alert, Mexican officials came out to defend public safety in the area, apparently concerned about a possible hit to tourism.
Forensic investigators in chemical warfare suits have been scouring the town for clues.
Authorities have raised concerns since a nerve agent was exposed to have poisoned a Russian former spy and his daughter. The senior intelligence officer with Russian military intelligence GRU, was pardoned in a spy swap in 2010 and settled in Salisbury, Wiltshire.
Theresa May, then the Home Secretary, told MPs she would be seeking European arrest warrants for the two suspected killers, and said there would be a Treasury freeze on the pair's assets. It would be a mistake for Western spy services to get back into the practice of political assassinations. In London, the Russian Embassy is acting like a boorish troll, tweeting "What a coincidence!" Skripal's wife, Liudmila, died of cancer at 60, in 2012.