Britain will methodically work out who carried out a nerve agent attack on a Russian ex-spy and his daughter in a quiet English city, then take action, interior minister Amber Rudd said on Thursday.
Former double agent Sergei Skripal, 66, and his daughter Yulia, 33, have been in hospital in a critical condition since they were found unconscious on Sunday on a bench outside a shopping centre in Salisbury.
British media and some politicians have speculated that the Russian state could be behind the attack - suggestions dismissed by Moscow as part of an anti-Russian campaign.
In a more measured tone, Rudd cautioned against jumping to conclusions before the facts were known.
"When we have all the evidence of what took place we will, if it is appropriate, attribute it to somebody," she said during an interview on BBC radio.
"If that is the case then we will have a plan in place. We need to be very methodical, take a cool head and be based on the facts, not on rumour," she said.
Police said on Wednesday that a nerve agent was used against Skripal and Yulia. A British police officer who was also harmed by the substance was now able to talk to people although he remained in a serious condition, Rudd said.
Scientific tests by government experts have identified the specific nerve agent used, which will help identify the source, but authorities have refused to disclose the details.
Skripal betrayed dozens of Russian agents to British intelligence before his arrest by Russian authorities in 2004. He was sentenced to 13 years in prison in 2006, and in 2010 was given refuge in Britain after being exchanged for Russian spies.
The attack on Skripal has been likened in Britain to the assassination of ex-KGB agent Alexander Litvinenko, a critic of Russian President Vladimir Putin, who died in London in 2006 after drinking green tea laced with radioactive polonium-210.
A British public inquiry later said Litvinenko's murder had probably been approved by Putin and carried out by two Russians, Dmitry Kovtun and Andrei Lugovoy, an ex-KGB bodyguard who later became a member of parliament.
Both men denied any responsibility and Russia has refused to extradite them to stand trial.
Rudd was pressed during her BBC interview on whether Britain had been too soft on Russia following the Litvinenko murder, sending out a message that such acts could be carried out with impunity.
She strongly denied this and hinted that if Russia turned out to be implicated in the attack on Skripal, action would be taken against it.
"We are absolutely robust about any crimes committed on these streets in the UK. There is nothing soft about the UK's response to any sort of state activity in this country," she said.
"We know exactly what we're doing. We are going to be waiting for the evidence. When we have it, if we attribute, we will have a plan."