The substance which has left two people fighting for their lives in Amesbury, Wiltshire, was the nerve agent Novichok, authorities have confirmed.
It is the same substance which was used to poison former Russian spy Sergei Skripal and his daughter Yulia earlier this year.
Here are some of the major questions around the case.
- What do the police need to do next?
The priority for the investigation is to establish how the pair, named locally as Dawn Sturgess and Charlie Rowley, have come into contact with the nerve agent, according to Neil Basu, Assistant Commissioner of Specialist Operations at Scotland Yard.
He revealed that around 100 detectives from the Counter Terrorism Policing Network are investigating, alongside colleagues from Wiltshire Police.
Mr Basu said a number of sites in Amesbury and Salisbury, areas which they think the pair visited before they fell ill, have been cordoned off.
In a move to reassure the public, he said there is "no evidence" that either of them recently visited any of the sites decontaminated following the attack on the Skripals.
Mr Basu said there will be an increased police presence, with officers wearing protective equipment as they work at a number of sites.
- What are the possible links?
A senior Government source said authorities had not been able to ascertain the item used to deposit the Novichok in the attack on the Skripals and it is possible the latest victims came into contact with that item.
This could raise the prospect that at least one other area in Salisbury city centre was contaminated with the nerve agent but had not previously been identified.
Mr Basu said investigators are "not in a position to say whether the nerve agent was from the same batch the Skripals were exposed to".
It will be up to scientists to determine if the Novichok was from the same batch which was used against the Skripals.
He added: "We know what the nerve agent is but we don't know what the transmission of it was."
Mr Basu said police do not have "any intelligence or evidence" that the couple were targeted in any way.
"There is nothing in their background to suggest that at all."
- What is the Government response?
A senior Government source told the Press Association the development would raise "serious questions of the oversight and choosing of the (clean-up) sites and how this was handled".
The clean-up was carried out by Defra and overseen by Environment Secretary Michael Gove under the guidance of the Home Office, according to a source.
The ongoing decontamination of a number of sites around Salisbury is reported to have cost millions of pounds so far.
Home Secretary Sajid Javid will chair a meeting of the Government's Cobra emergencies committee on Thursday.
Cobra meetings are used for national crises with the aim of making swift and direct decisions after talks between senior politicians, military officials and intelligence agencies, depending on the situation.
- What is Public Health England's advice?
Professor Dame Sally Davies, Chief Medical Officer for England, said the Skripal episode meant officials had a "well-established response" in place.
She said: "You do not need to seek advice from a health professional unless you are experiencing symptoms."
Public Health England issued "highly precautionary" advice to anyone who visited five locations identified by police between 10pm last Friday and 6.30pm on Saturday:
- Wash clothes you were wearing in an ordinary washing machine using regular detergent at a normal temperature.
- Wipe items such as phones and handbags with cleansing or baby wipes and dispose of the wipes in the bin.
- Keep your items double-bagged and securely fastened, if they are dry-clean only. Further advice will follow.
- Items such as jewellery and glasses should be hand-washed with warm water and detergent, then rinsed with cold water.
- Should people be worried?
The risk to the general public remains low, Dame Sally said.
Anyone who had been possibly exposed to the same source of contamination would by now be suffering severely.
Contaminated sites in Salisbury which had been at the centre of Novichok fears, such as The Maltings, are completely safe, Mr Basu said.