Forensic analysis concluded that ex-spy Sergei Skripal and his daughter Yulia fell suddenly ill after they were exposed to a nerve agent on Sunday evening, Rudd told the House of Commons, adding it was too early to speculate who orchestrated the crime, Efe news agency reported.
She said that the first police officer to respond to the emergency -- who also became seriously ill through exposure to the toxic substance -- remained in intensive care but had now regained consciousness.
"The use of a nerve agent on UK soil is a brazen and reckless act," Rudd said. "This was attempted murder in the most cruel and public way," she added, saying that people were right to want to know who to hold to account.
"But, if we want to be rigorous in this investigation, we must avoid speculation and allow the police to carry on their investigation," she went on to say.
In 2006, former Russian army colonel Skripal was convicted of high treason by a Moscow court for passing on state secrets to British intelligence, but he was granted asylum in the UK four years later as part of a prisoner swap.
This latest attack on British soil harbored echoes of the 2006 assassination in London of Russian defector Alexander Litvinenko.
A UK public inquiry concluded that Russian President Vladimir Putin had authorised Litvinenko's murder, a finding that the Kremlin has vehemently denied.
Russia has also denied having any information on the Skripal case.
Rudd said UK counter-terror units were following hundreds of leads in order to trace the perpetrators of the nerve agent in Salisbury, a picturesque and normally tranquil cathedral city in the county of Wiltshire.
Rudd did not reveal the type of nerve gas that was used in the assault, but earlier told the press that it was "very rare".
Sergei and Yulia Skripal were found slumped on a park bench in Salisbury on Sunday, prompting a huge emergency incident as police closed down a nearby restaurant where they are thought to have eaten that evening and also a pub where they had a drink.
Both victims remained unconscious, in a critical but stable condition, Rudd said.
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