A police officer is fighting for his life, Scotland Yard has revealed, as they confirmed that the "substance" used against Russian spy Colonel Sergei Skripal and his daughter was a nerve agent.
Metropolitan Police Assistant Commissioner Mark Rowley made the announcement, hours after the Home Secretary suggested investigators were close to a breakthrough, the Daily Telegraph reports.
Rowley said the poisoning was being treated as attempted murder and said the Russian pair had been "targeted specifically".
"This is being treated as... attempted murder by administration of a nerve agent.
"These two people remain critically ill in hospital.
"A police officer who was one of the first to attend the scene is now also in a serious condition in hospital.
"Wiltshire Police are, of course, providing every support to his family.
"While we are now in a position to confirm that their symptoms are as a result of exposure to a nerve agent, I will not be providing further information at this stage about the exact substance, which has been identified."
The development came hours after a meeting of the British government's emergency committee.
After the meeting, Home Secretary Amber Rudd said Scotland Yard will be revealing more details on the Salisbury investigation later today.
She had been chairing a meeting of Cobra in Whitehall to discuss the crisis as Skripal, 66, and his 33-year-old daughter, Yulia, continue to fight for their lives in hospital.
Rudd said: "This is likely to be a lengthy and ongoing process. We need to make sure that the police and the other services have the space to continue that investigation.
"We need to keep a cool head and make sure that we collect all the evidence we can and we need to make sure that we respond not to rumour but to all the evidence that they collect and then we will need to decide what action to take."
Scotland Yard counter-terror officers further extended cordons in the city overnight, while scientists at Porton Down, the UK's military research facility, continue their investigation.
Earlier, emergency services were to an office in central Salisbury after reports that a person had been taken seriously ill.
Three fire engines and at least two ambulances arrived at offices close to Zizzi's restaurant on Castle St, Salisbury.
Several police officers and paramedics were seen inside Sarum House, next door to the pizzeria.
That the substance could be a nerve agent was one theory being explored. The deadly nerve agent, VX, was used last year in the murder of Kim Jong Nam, the half-brother of North Korean leader Kim Jong Un.
The high-profile investigation is entering its fourth day after Skripal and Yulia were found unconscious on Monday (NZT).
They are thought to have been exposed to the substance as they sat on a bench outside a shopping centre.
Detectives urged anybody who visited Salisbury town centre on that day or who visited a Zizzi restaurant or the Bishop's Mill pub to come forward.
A bridge in Salisbury, Wiltshire, was shut, and police were seen at Zizzi restaurant, where the pair are said to have eaten.
The home affairs select committee has called for a review into 14 deaths that were not treated as suspicious by UK police but were reportedly identified by US Intelligence officials to Buzzfeed as linked to Russia.