British counterterrorism police were investigating Wednesday after two people were left in critical condition, exposed to an unknown substance a few miles from where a former Russian spy and his daughter were poisoned with a nerve agent.

The Wiltshire Police force declared a “major incident” after a man and a woman in their 40s were hospitalized after being found unconscious at a residential building in Amesbury, eight miles (13 kilometers) from Salisbury, where Sergei and Yulia Skripal were poisoned on March 4.

London’s Metropolitan Police force said “given the recent events in Salisbury,” counterterrorism officers were working with local police on the investigation.

Police cordoned off the building and other places the two people visited before falling ill, but health officials said there was not believed to be a wider risk to the public.

The man and woman were sent Saturday to Salisbury District Hospital, where the Skripals spent weeks in critical condition after being poisoned. Police said authorities initially believed the latest victims might have taken a contaminated batch of heroin or crack cocaine.

“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 in a statement. “At this stage, it is not yet clear if a crime has been committed.”

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 U.K. as part of a 2010 prisoner swap. He had been living quietly in Salisbury, a cathedral city 90 miles (145 kilometers) southwest of London, when he was struck down along with his 33-year-old daughter Yulia, who was visiting him.