The British government has described the use of a nerve agent to attack a Russian ex-spy and his daughter as a “brazen and reckless” act.
However, Home Secretary (interior minister) Amber Rudd warned against speculation over who was behind it.
UK investigators are working to establish the source of a nerve agent they believe was used to try to kill Sergei Skripal, 66, and his daughter Yulia, 33. The pair were found unconscious on Sunday on a bench outside a shopping centre in Salisbury, southern England.
“The use of a nerve agent on UK soil is a brazen and reckless act. This was attempted murder in the most cruel and public way,” Rudd told the House of Commons in a statement.
"But if we are to be rigorous in this investigation, we must avoid speculation, and allow the police to carry on their investigation. We will respond in a robust and appropriate manner once we ascertain who was responsible."
Rudd said Britain was committed to doing everything possible to bring the perpetrators to justice.
Earlier, she said that a police officer who was one of the first on the scene in Salisbury was conscious and able to speak, although his condition "remains serious".
On Thursday the Scotland Yard confirmed to NBC that there is a police cordon at the gravestone of Skripal's wife and son. However they declined to give any further information.
On Wednesday counter terrorism police confirmed the case was being treated as attempted murder.
"In summary, this is being treated as a major incident involving attempted murder by administration of a nerve agent," Mark Rowley, the head of counter terrorism policing, said on Wednesday as police gave their latest update on the situation.
"I can also confirm that we believe the two people originally who became unwell were targeted specifically," he added.
Rowley declined to give details of the substance used, as it was part of the investigation. However, police say government scientists have identified the nerve agent.
Hundreds of detectives are now working on the Skripal case to build up a timeline of the former agent’s movements over the last few days, Rowley said.
The area where the father and daughter were found remains sealed off – including the Zizzi pizza restaurant where they dined and the Bishop’s Mill pub where they had a drink. Cordons have been set up near a business park in the nearby town of Amesbury.
England’s chief medical officer said the incident posed a low risk to the wider public and anyone feeling unwell was advised to seek medical advice.
Although parallels have been drawn with previous cases, such as the poisoning of ex-KGB agent Alexander Litvinenko in 2006, some experts suggest there are differences in the Skripals’ situation.
“I’m not aware of a nerve agent having been used in this way previously,” said Malcolm Sperrin, a professor at the Institute of Physics and Engineering in Medicine. Nerve agents could cause heart failure, respiratory arrest, twitching or spasms, he said.
The UK interior minister's caution on Thursday contrasted with the earlier words of the foreign secretary.
On Wednesday Boris Johnson renewed his warning that the UK would respond “robustly” if the poisoning was found to be a hostile act from another country. The previous day he said if Moscow were behind the affair then Britain could look again at sanctions and take other measures to punish Russia, describing it as a “malign and disruptive” state.
Russia slammed his comments as “wild” and says anti-Russian hysteria is being whipped up intentionally to damage relations with London. Moscow denies involvement.
Russian Foreign Ministry spokeswoman Maria Zakharova told reporters in Moscow on Wednesday that the speculation was “provocative black PR”.
Reuters has quoted an anonymous US security source as saying that the main line of police inquiry was that Russians may have used the substance against Skripal in revenge for his treachery.
The former agent betrayed dozens of Russians to British intelligence before his arrest by Russian authorities in 2004.
He was sentenced to 13 years in prison in 2006 after a secret trial. In 2010 he was given refuge in Britain after being exchanged for Russian spies caught up in the West as part of a Cold War-style spy swap at Vienna airport.
Last year Sergei Skripal’s son, Alexander, died aged 43. UK media have said his death occurred during a visit to St Petersburg. Skripal’s wife Liudmila died of cancer in 2012. Both are buried in Salisbury.
Comparisons with Litvinenko
Britain has compared the Skripal’s case with the Litvinenko murder. The former KGB agent was killed with the radioactive polonium-210 which had been added to his tea at London’s Milennium Hotel.
A British inquiry said Russian President Vladimir Putin probably approved Litvinenko’s murder and that it was hatched by the Federal Security Service (FSB), the main successor to the Soviet-era KGB.
Russia denied involvement and the case damaged relations between the two countries for several years.