Britain's counter-terrorism chief said on Wednesday that two people who are critically ill in hospital in Wiltshire were exposed to the Novichok nerve agent, the same toxin that was used in an attack on ex-Russian spy Sergei Skripal.
Neil Basu said there was nothing in the background of the pair to suggest they would be a target. He also told reporters police did not yet know how the Novichok nerve agent was transmitted.
The man and woman, both British and in their 40s, were found in critical condition at a house in Amesbury on Saturday, 13 kilometers (eight miles) from Salisbury where Skripal and his daughter were poisoned with nerve agent on March 4.
England's Chief Medical Officer Sally Davies said Wednesday the risk to the general public remains low despite the incident.
Medical officials said people who had been in the area where the couple had been should take precautions and wash their clothes.
Police cordoned off places the two people visited before falling ill, but public health officials say there is not believed to be a wider risk. They were hospitalized Saturday and authorities at first believed they might have taken a contaminated batch of heroin or crack cocaine.
"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. "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.
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.
Counter-terrorism teams from London's Metropolitan Police were called in to help local forces in Wiltshire at the time of the Skripal poisoning. On Wednesday, however, Scotland Yard referred media calls to the Wiltshire police.
The statement from Wiltshire Police came only a month after police from 40 departments in England and Wales returned home after months of working on the Skripals' poisoning. The Wiltshire Police force spent about 7.5 million pounds ($10 million) dealing with the aftermath of the Skripals' poisoning and believe that his front door was contaminated with the nerve agent.
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 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 don't know what their long-term prognosis is.
The Skripals have been taken to an undisclosed location for their protection.