What is known so far?
Skripal and his daughter Yulia were found unconscious on a bench in Salisbury on Sunday.
Rudd says a police officer who treated them is in serious but stable condition and is "talking and engaging". It was not clear who they are or whether they are linked to the case.
Hundreds of investigators, led by counter-terror police, are working to find out who is responsible for what is feared to be a sophisticated plot amid heightened tensions between Britain and Russian Federation.
Rudd declined to say if she believed Russian Federation was behind the attack, but says Britain will "if it is appropriate, attribute it to somebody".
Tom Tugendhat, chairman of the Foreign Affairs Select Committee, has said all the evidence points to the nerve agent attack being ordered by the Kremlin. She declined to give details of the police investigation.
He remained in a serious condition although he is now able to talk, Ms Rudd said. "We'll do that properly, at the right time, and on the basis of the best evidence". Health officials insist there has only been a low risk to the public.
What makes this particular attack so sinister, and why speculation about who is responsible has gripped the nation, is that nerve agents are incredibly hard to come by, and are ordinarily developed by States.
Police have cordoned off the bench where the pair were found, as well as an Italian restaurant and a pub they visited before their collapse.
That statement came after the official number of people possibly exposed to the nerve agent used in the assassination attempt climbed to 21.
Citing unnamed security sources, The Sun newspaper said Wednesday they were poisoned by a very rare nerve agent which only a few laboratories in the world could have produced.
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"We would like to hear from anybody who visited the area close to The Maltings shopping center where these two people were taken ill on Sunday afternoon, and may have seen something that could assist the investigation", Rowley said.
Russian Federation denounced the distortions by the United States about the organization of a meeting between Foreign Minister Sergei Lavrov and Secretary of State Rex Tillerson in Ethiopia. He also taught at the Russian Defence Ministry's defense academy in Moscow.
He had been convicted in his home country in 2006 for passing state secrets to MI6. He is thought to have done serious damage to Russian spy networks in Britain and Europe. Russian media said his motivation was financial: he received more than $100,000 paid into a Spanish bank account.
Penkovsky, a friend of the then GRU chief, informed British and American spies of a Moscow operation to place nuclear missiles in Cuba.
Penkovsky was arrested in 1962 and executed in 1963 after being found guilty of high treason and espionage.
Thirty-three-year-old Yulia recently arrived in Britain to meet her 66-year-old father.
Dubbed the "illegals", some of the Russians had spent years quietly collecting information and trying to meet Americans with political ties. (AP/AAP) A memorial plaque for Alexander Skripal, the son of ex Russian spy Sergei Skripal.
The swap, one of the biggest since the Cold War ended in 1991, took place on the tarmac of Vienna airport where a Russian and a US jet parked side by side before the agents were exchanged.
The Russian Embassy also denounced how President Piotro Poroshenko is described as a hero, after Moscow denounced the injustice committed by the Arbitration Court in Stockholm when it ruled that Gazprom was in debt to Ukraine.
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