The exposure of an apparently random British couple to the same nerve agent used against former Russian double agent Sergei Skripal, in the same region, has sent officials scrambling to discover the source of the contamination.
Here is what we know about the latest incident.
Police said samples from both patients, who fell ill on Saturday, were sent to defence laboratory Porton Down on Monday due to concern over their symptoms.
Police said the tests confirmed that the substance was Novichok, a Soviet-designed nerve agent, although police could not confirm it was the same batch used against Skripal on March 4.
The local Wiltshire police force initially suspected the couple had fallen ill after using heroin or crack cocaine from a contaminated batch of drugs.
Counter-terrorism police took over the investigation on Wednesday after the exposure to Novichok was confirmed.
Counter-terrorism police chief Neil Basu has said there in no evidence to suggest that the man and the woman, named locally as Charlie Rowley and Dawn Sturgess, "were targeted in any way".
Government officials have said the two may have been exposed to the poison accidentally following the previous attack on the Skripals.
Risk to the public
The police chief insisted that there was "a low risk to the general public," but the apparent randomness of the contamination raises many unanswered questions for residents of Salisbury, which was only just returning to normal after finding itself the epicentre of international tensions.
Despite urging calm, England's chief medical officer Dame Sally Davies gave a specific warning on Thursday, telling the public "to be careful of picking up any unknown or already dangerous objects such as needles and syringes".
The couple remain in a critical condition in Salisbury District Hospital, which also treated Skripal, his daughter Yulia and police officer Nick Bailey, who came to assist them.
All three eventually survived, with Sergei Skripal the last to leave hospital, on May 18.