Counter-terror police are working to unravel what is now feared to be a sophisticated chemical weapon plot targeting a Russian spy and his daughter.
A nerve agent is believed to have been used to critically injure Sergei Skripal, 66, and 33-year-old Yulia in Salisbury, Wiltshire, on Sunday.
One of the first police officers to arrive at the bench where the pair were slumped is also seriously ill in hospital.
It remains unclear who is responsible for poisoning the pair, but the attack has stoked tensions between Britain and Russia amid suspicions of state responsibility.
Assistant Commissioner Mark Rowley, the head of counter-terrorism policing, revealed on Wednesday that the incident was being treated as attempted murder and the pair had been "targeted specifically".
He declined to specify the nerve agent or how it was administered.
He said: "Having established that a nerve agent was the cause of the symptoms, leading us to treat this as attempted murder, I can also confirm that we believe the two people who originally became unwell were targeted specifically.
"Our role now of course is to establish who is behind this and why they carried out this act."
Hundreds of detectives, forensic officers and analysts are working on the case, which has drawn comparisons to the poisoning of former Russian intelligence officer Alexander Litvinenko on British soil in 2006.
Nerve agents, which are chemical weapons, have been used in assassinations and attacks in war zones in recent years.
Kim Jong Un's half-brother Kim Jong Nam was killed at an international airport in Malaysia last year in an attack using a nerve agent known as VX.
Another well-known nerve agent, sarin gas, killed more than 90 people in a rebel-held area in Khan Sheikhoun, Syria, drawing international condemnation of the Bashar Assad regime.
Access to such toxins are tightly regulated, meaning the Salisbury plot would have taken considerable planning to execute.
Mr Rowley reiterated his appeal for anyone who was in Salisbury city centre on Sunday to come forward to help with the "missing pieces" in the case.
Dame Sally Davies, the chief medical officer, said the incident posed a "low risk" to the public and advised that all the sites the par were known to have visited had been "secured".
Public Health England later confirmed it had contacted all first responders who had attended the scene.
Russia have denied responsibility for the attack, which comes seven years after Mr Skripal was released from the country as part of a spy swap with the US.
He had been convicted in his home country in 2006 for passing state secrets to MI6.
The investigation has triggered a diplomatic row and prompted crisis talks in Whitehall but Home Secretary Amber Rudd said police must respond to "evidence, not to rumour".
It comes as police extended the cordons in Salisbury city centre, and also sealed off part of a business park in nearby Amesbury.
The Defence Science and Technology Laboratory in nearby Porton Down, which has state-of-the-art equipment to look for trace amounts of substances, is believed to have been involved in examining the substance.