INVESTIGATORS are racing to find an item contaminated with the deadly nerve agent Novichok that has left two people critically ill in Wiltshire.

Here are some key questions answered about the hunt for the potentially-deadly object that could also provide vital evidence in the Salisbury poisoning probe.

What are investigators looking for?

Police have said they believe Dawn Sturgess and Charlie Rowley fell ill with Novichok poisoning after handling a "contaminated item".

What could the item be?

Speculation over what the item is has ranged from a tainted cigarette butt to a discarded container like a phial or syringe used to transport the nerve agent for the attack on Sergei and Yulia Skripal.

What do experts suspect?

After some four months passing between the incidents some experts believe it would be unlikely the poison would have remained so potent if it has been left exposed to the elements. One expert told the BBC it was more likely the poison had stayed protected in a closed space. However other experts say the nerve agent was designed to be persistent and not break down.

How did the victims become contaminated?

Nerve agents, including Novichok, can be inhaled as a fine powder, absorbed through the skin or ingested.

Where did the victims first come into contact with it?

Authorities are working on the strong assumption that they came into contact with the nerve agent in a different location to the sites which were part of the cleanup after the Skripal incident in Salisbury. Six sites visited by Mr Rowley and Ms Sturgess in Salisbury and Amesbury before they fell ill have been cordoned off.

Where are the sites that have been sealed off?

The sites in Salisbury include the Queen Elizabeth Park and Mrs Rowley's flat in John Baker House in the city centre. In Amesbury they include Mr Rowley's home on Muggleton Road, a Boots chemist and a baptist church.

Where are inquiries focused?

According to reports, investigators are concentrating their search on the homes of Ms Sturgess and Mr Rowley amid fears they picked the item up and took it back to either one of the properties. The couple were both taken to hospital after falling ill at Mr Rowley's home on Saturday. John Baker House was evacuated on Thursday, while Mr Rowley's home has been sealed off.

What have the public been told?

Police have been unable to rule out the possibility of another incident involving the poison. Locals have been advised to not pick anything up in public places in order to stay safe.