Search widens for source of Novichok nerve agent in Amesbury A man and a woman are both in critical condition.

LONDON -- Wider cordons have been set up around the town of Amesbury in Wiltshire, England, along with metal barriers protecting hazardous material tents, after a couple was believed by authorities to have been poisoned by a nerve agent last Saturday.

The man and the woman are both in critical condition and are receiving treatment at Salisbury District Hospital, where former Russian double-agent Sergei Skripal and daughter Yulia were treated after they were poisoned by a nerve agent in March in Salisbury.

British investigators believe the Amesbury pair have been infected by the same nerve agent, a Soviet-developed compound from a group of nerve agents called Novichok.

The Associated Press has identified the couple as Dawn Sturgess, 44, and Charlie Rowley, 45, whose identities have not been released by authorities.

“Based on the information we received from a friend of the couple who had been with them that day, alongside the initial medical diagnosis and items located at the address, there was a strong indication from this that the decline in their health may have been connected to a contaminated batch of illegal drugs," Wiltshire Police said in statement. "Based on this our initial hypothesis was that the situation was connected to a contaminated batch of illegal drugs.”

Police do not believe that the Amesbury pair were deliberately targeted but are still investigating whether there is a link to the Salisbury poisoning.

Authorities are searching several locations in Amesbury and Salisbury, which are about 9 miles apart, for where the pair may have come across nerve agents. Officials are working to determine whether they were exposed to a new batch of Novichok or whether they stumbled onto the same materials used to poison the Skripals.

The search to determine the means and locations of the latest exposure has compelled police to close off additional locations around Wiltshire, including a pharmacy, a park, a church and several areas in Salisbury such as Queen Elizabeth Gardens and a homeless shelter.

A March assessment by the British government found that the Russian state was most likely responsible for the Skripals’ poisoning. The Kremlin has consistently denied the British accusations.

British Home Secretary Sajid Javid Thursday urged Russia to explain the latest incident in Wiltshire, saying the U.K. “will stand up to the actions that threaten our security.”

Vil Mirzayanov, a Soviet scientist who worked on the development of Novichok before defecting to the United States, said that if the Amesbury couple were infected by material left over from the Skripals incident, it is unlikely the agent was exposed to the elements because it retained enough potency to cause harm.

If the Novichok was from a previous batch, Mirzayanov told ABC News, it would have had to have been stored in a secure vial or container, which may have been picked up by the Amesbury couple and caused them to fall ill.

The agent is so potent that only a small amount would be needed to cause death, he said, adding it could remain deadly after several years if stored in a secure environment.