British police declared on Wednesday a major incident after two people were left in critical condition due to their exposure to an unknown substance near the town where a former Russian spy and his daughter were poisoned by a nerve agent in March. Police in Wiltshire said a man and a woman were found unconscious on Saturday in Amesbury. Amesbury lies seven miles (11 kms) to the north of Salisbury, where the Russians were found slumped unconscious on a bench on March 4. Counter-terrorism officers were called in for the investigation, Londons police force said. "Given the recent events in Salisbury, officers from the counter terrorism network are working jointly with colleagues from Wiltshire Police regarding the incident in Amesbury," the Metropolitan Police said in a statement. "As Wiltshire Police have stated, they are keeping an open mind as to the circumstances surrounding the incident." The pair, both in their 40s, were initially believed to have taken heroin or crack cocaine from a contaminated batch of drugs. They are being treated at Salisbury District Hospital, which remains open as usual, police said. “However, further testing is now ongoing to establish the substance which led to these patients becoming ill and we are keeping an open mind as to the circumstances surrounding this incident,” police said on Tuesday. “At this stage it is not yet clear if a crime has been committed. A police investigation has been established. In addition, a full multi-agency response has been coordinated.” The couple are showing "very similar" symptoms to those suffered by Russian former double agent Sergei Skripal and his daughter Yulia, the Sun newspaper reported on Wednesday. "Doctors have determined that the pair have been poisoned by toxins and are showing very similar ... symptoms to the former Russian military intelligence colonel and his daughter," the newspaper said. Police said sites in both Amesbury and Salisbury that they believed the man and woman found in Amesbury had frequented would be cordoned off as a precaution. A Public Health England (PHE) spokesman said there was not a significant risk to the wider public. A major incident is a designation allowing British authorities to mobilize more than one emergency agency. Sergei Skripal, 66, is a former Russian intelligence officer who was convicted of spying for Britain before coming to the UK as part of a 2010 prisoner swap. He had been living quietly in Salisbury, a cathedral city located 90 miles (145 kilometers) southwest of London, when he was struck down along with his 33-year-old daughter Yulia. After being found unconscious in the street, the two spent weeks in critical condition at the hospital. Doctors who treated the Skripals said at the time said they expected them to die. They say they still do not know what their long-term prognosis is. The Skripals have been taken to an undisclosed location for their protection. Britain accuses Russia of poisoning the Skripals with a Novichok nerve agent, a group of chemical weapons developed by the Soviet Union during the Cold War. Moscow denies the allegation. The poisoning sparked a Cold War-style diplomatic crisis between Russia and the West, including the expulsion of hundreds of diplomats from both sides.