Health officials have said the risk to the public is low after it was revealed a couple in Wiltshire have been left in a critical condition following exposure to the nerve agent聽Novichok.
Five locations in Salisbury and Amesbury have been identified by police as part of their investigation, but Public Health England (PHE) said there is 鈥渘o immediate鈥� danger to those who may have visited them.
Mike Wade, deputy director of health protection for PHE South West, said: 鈥淥ur current advice, based on the small number of casualties affected, is that the risk to the public is low.
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鈥淲e will keep this assessment under constant review as further information becomes known.鈥�
It was initially believed that the two patients fell ill after possibly using drugs from a contaminated batch, police have said.
But after further tests, authorities declared a major incident and on Wednesday night counter-terror police assumed responsibility for the investigation after the Government鈥檚 Porton Down laboratory concluded that the pair had been exposed to Novichok.
A senior Government source told the Press Association it is believed there was cross-contamination of the same batch of nerve agent involved in the Salisbury attack, as opposed to a secondary attack.
鈥淭hey (the authorities) have never been able to ascertain the item used to deposit the Novichok and it鈥檚 possible the pair have come into contact with that item,鈥� the source said.
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.
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 鈥渘o evidence鈥� that either of them recently visited any of the sites decontaminated following the attack on the Skripals.
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.
Basu said investigators are 鈥渘ot in a position to say whether the nerve agent was from the same batch the Skripals were exposed to鈥�.
It will be up to scientists to determine if the Novichok was from the same batch which was used against the Skripals.
He added: 鈥淲e know what the nerve agent is but we don鈥檛 know what the transmission of it was.鈥�
Mr Basu said there was nothing in the couple鈥檚 background to suggest they were targeted directly.
Was it the same batch?
Novichok nerve agents 鈥� also known as the 鈥淣-series鈥� 鈥� were secretly developed by the former Soviet Union beginning in the 1970s.聽 They are binary agents, meaning they are made from two precursor chemicals that are mixed together just before use. And they remain potent for a long time.
鈥淐onfirmation that this was the same chemical agent that poisoned the Skripals really confirms a lot of things that we believed were true about this 鈥榥ovichok鈥� class of nerve agents.聽 They are designed to be quite persistent 鈥� they hang around in the environment, neither evaporating or decomposing quickly,鈥� says Andrea Sella, Professor of Inorganic Chemistry, UCL.
鈥淭hat means that if a container or a surface was contaminated with this material it would remain a danger for a long time and it will be vital to trace the movements of this couple to identify where they might have come into contact with the source.聽 So while the public at large are at very low risk from this material, until the source is found there is a remote chance that someone else might come into contact with it.鈥�
What is the Government response?
A senior Government source told the Press Association the development would raise 鈥渟erious questions of the oversight and choosing of the (clean-up) sites and how this was handled鈥�.
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鈥檚 Cobra emergencies committee today.
What is Public Health England鈥檚 advice?
Professor Dame Sally Davies, Chief Medical Officer for England, said the Skripal episode meant officials had a 鈥渨ell-established response鈥� in place.
She said: 鈥淵ou do not need to seek advice from a health professional unless you are experiencing symptoms.鈥�
Public Health England issued 鈥渉ighly precautionary鈥� advice to anyone who visited five locations identified by police between 10pm last Friday and 6.30pm on Saturday:
鈥� Wash clothes you were wearing in an ordinary washing machine using regular detergent at a normal temperature.
鈥� Wipe items such as phones and handbags with cleansing or baby wipes and dispose of the wipes in the bin.
鈥� Keep your items double-bagged and securely fastened, if they are dry-clean only. Further advice will follow.
鈥� 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.
