AMESBURY, England (Reuters) - Two British citizens are critically ill after they were exposed to Novichok, the same nerve agent that struck down a former Russian agent and his daughter in March, Britain’s top counter-terrorism officer said on Wednesday.

The pair, a local 44-year-old woman and a 45-year-old man, were hospitalized after being found unwell on Saturday in Amesbury, just miles away from Salisbury where ex-double agent Sergei Skripal and his daughter Yulia were attacked in March.

“I have received test results from Porton Down (military research center) which show that the two people have been exposed to the nerve agent Novichok,” Neil Basu, Britain’s most senior counter-terrorism officer, told reporters.

Britain has accused Russia of poisoning the Skripals with Novichok - a nerve agent developed by the Soviet military during the Cold War - in what is the first known offensive use of such a chemical weapon on European soil since World War Two.

Russia has denied any involvement in their poisoning.

UK counter-terrorism police are now leading the investigation, though Basu said it was unclear how the two people came into contact with the nerve agent or whether they had been specifically targeted.

“I don’t have any intelligence or evidence that they were targeted in any way,” Basu said. “There is nothing in their background to suggest that at all.”

Amesbury is located seven miles (11 km) north of Salisbury, where Skripal - a former colonel in Russian military intelligence who betrayed dozens of agents to Britain’s MI6 foreign spy service - and his daughter were found slumped unconscious on a bench on March 4.

Around 100 counter-terrorism officers are working on the case and police have cordoned off at least five different areas, including a park and a property in Salisbury, as well as a pharmacy and a Baptist church community center in Amesbury.

The March attack prompted the biggest Western expulsion of Russian diplomats since the Cold War as allies in Europe and the United States sided with Prime Minister Theresa May’s view that Moscow was either responsible or had lost control of the nerve agent.

Mystery surrounds the attack and the motive is unclear, as is the logic of using such an exotic nerve agent which has overt links to the Soviet military during the Cold War.

Russia, which is currently hosting the soccer World Cup, has denied any involvement in the March incident and suggested Britain had carried out the attack to stoke anti-Moscow hysteria.

Moscow also hit back by expelling Western diplomats, questioning how Britain knows that Russia was responsible and offering rival interpretations, including that it amounted to a plot by British secret services. Russian officials questioned why Russia would want to attack an aging turncoat who was pardoned and then traded in a Kremlin-approved 2010 spy swap.

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NOVICHOK AGAIN

Health chiefs said on Wednesday the risk to the public was low, though the exposure of two people apparently unconnected to espionage or the former Soviet Union will stoke fears that traces of the nerve agent remain in the area.

“As the country’s chief medical officer, I want to reassure the public that the risk to the general public remains low,” England’s Chief Medical Officer Sally Davies told reporters.

Slideshow ( 10 images )

Prime Minister May’s spokesman said the government’s emergency response committee had met to discuss the incident. Home Secretary Sajid Javid will chair a meeting of the emergency response committee on Thursday.

“The Amesbury investigation is ongoing and the police must be given the space they need to continue establishing the full facts,” Javid said.

“My thoughts at this time are with the two individuals affected. The government’s first priority is for the safety of the residents in the local area, but as Public Health England has made clear, the risk to the general public is low.”

After the Skripal poisoning, police investigators in protective hazmat suits scoured the ancient English cathedral city of Salisbury. Basu cautioned that police in protective clothing would return to the area.

Paramedics were called on Saturday morning to a house in Amesbury after the woman, named by media as Dawn Sturgess, collapsed and returned later in the day when the man, Charlie Rowley, also fell ill.

The pair, who are being treated at Salisbury District Hospital, were initially believed to have taken heroin or crack cocaine from a contaminated batch, police said.

But tests showed they had been poisoned with Novichok.

“We are not in a position to say whether the nerve agent was from the same batch that the Skripals were exposed to,” Basu said. “The possibility that these two investigations might be linked is clearly a line of enquiry for us.”

The hospital is where the Skripals also spent weeks in a critical condition before slowly recovering and being discharged.

Yulia told Reuters in May: “We are so lucky to have both survived this attempted assassination. Our recovery has been slow and extremely painful.”

Russia has said it does not possess such nerve agents, did not develop Novichok, and President Vladimir Putin dismissed as nonsense the notion that Moscow would have poisoned Skripal and his daughter.