QUESTIONS about how two more people have come to be poisoned with a nerve agent in Salisbury are still going unanswered.

Four months ago city residents were assured the risk to public health was low and that all sites potentially contaminated with Novichok had been secured by the police.

But since Saturday cordons have appeared in Salisbury and Amesbury, and news emerged that residents Charlie Rowley and Dawn Sturgess were critically ill at Salisbury District Hospital, after coming into contact with the same nerve agent used to poison Sergei and Yulia Skripal in March.

At a public meeting at Salisbury City Hall last night (Thursday) Professor John Simpson of Public Health England (PHE) said the risk was deemed to be low because during each incident only a small number of people have required treatment.

Professor Simpson said PHE are constantly updating their risk assessments as new information comes to light, but the number of casualties "suggests that there was a very small amount of material [Novichok]".

The Journal asked Chief Constable Kier Pritchard how he could be sure that there would be no more contaminated sites, given the fact the public were assured in March that all potentially unsafe areas had been identified.

"Of course, it's that question that sits at the heart of everybody's thinking," he responded, adding that he "fully understands the level of anxiety" caused by the latest incident.

"With the best information, with the available facts that were provided, the locations that we secured were deemed to be those that were fully adequate at that time."

When asked how the public could be reassured that this incident would not be repeated in months to come, he said: "I hope we are not back in this position in four months' time. Salisbury was recovering really well."

Security minister Ben Wallace said the "working assumption" is that the pair were exposed to Novichok as a result of the previous incident, rather than a separate attack.

And home secretary Savid Javid warned the public should not pick up any unknown objects in the city.

Last night CC Pritchard said it was unknown where the pair had been contaminated, whether it was Salisbury or Amesbury, but that the "strong working assumption" was that the couple came into contact with the agent at a site not related to the first incident.

"It’s too early for us to be able to understand that [where the contamination occurred]," he said.

"We simply do not know. That’s the question we’re all looking for the answer for”