Around 180 troops were deployed in Salisbury following the nerve agent attack on Sergei Skripal and his daughter Yulia.
Military personnel have been called in to assist in the investigation.
Police have cordoned off Skripal's modest home in Salisbury, about 130 kilometres from London, and erected forensic tents in the garden.
Police officer Nick Bailey, who was at the scene on Sunday, has been discharged from intensive care.
The two remain in critical condition in a Salisbury hospital, while authorities say Bailey is making progress toward recovery. The agent had been identified but he declined to give any more details about it.
"There will be a time to have answers", Rudd said, adding police should be allowed the "space they need to go through the area carefully".
Former London police chief Ian Blair said Friday that Bailey had visited Skripal's house - perhaps a hint that the nerve agent may have been delivered there. The officer - I'm very sorry he's been injured - has actually been to the house, whereas there's a doctor who looked after the patients in the open who hasn't been affected at all. "There may be some clues floating around in here". He later admitted that in 1995 he was recruited by the British secret services.
Skripal, a retired Russian military intelligence colonel, was one of four prisoners released by Moscow in 2010 in exchange for 10 Russian sleeper agents in the United States, as part of a swap which included high-profile spy Anna Chapman.
Those branded enemies of the Russian state have sometimes died mysteriously overseas, and the Skripal case echoes the death of Alexander Litvinenko, a former Russian agent who was poisoned in London in 2006 with radioactive polonium-210. The Kremlin has denied any involvement in the incident and says anti-Russian hysteria is being whipped up by the British media. They said health advice remains the same - that there is no broader risk to the public. "But the British government has remained extremely firm on sanctions and on a relatively anti-Russian position".
She said it was an "outrageous crime" and using a nerve agent on British soil was a "brazen and reckless act" that was "attempted murder in the most cruel and reckless way".
British Home Secretary Amber Rudd said proof was needed before jumping conclusions about what happened to the 66-year-old former spy, but that action would be taken once authorities knew exactly what had happened.
In her Commons statement, Ms Rudd said she would not comment further on the nature of the nerve agent as it is a "fast-paced criminal investigation". "That is not serious", he said.
Specialist investigators wearing protective suits have been seen examining the bench that Mr Skripal and his daughter collapsed on.
"Look, we have - our relationship with Russian Federation is not at a point where it looks like it is going to be normalized anytime soon". That is not serious.
"This is propaganda plain and simple, and it is trying to raise tensions, " Lavrov said.
On Thursday, the Home Secretary revealed a police officer who rushed to help is awake and talking in hospital.
Lavrov has been quoted as saying by state news agency Tass that "whether it's poisoning of some British subjects, whether it's rumors about interference in the USA election campaign, if assistance really is needed, then we are ready to consider its possibility".
Russian President Vladimir Putin "has developed deniability into an art form", said Fraser Nelson, editor of Britain's Spectator magazine, in a column Friday. Lugovoy is a former KGB bodyguard who later became a member of the Russian parliament.