British police say two Britons who fell critically ill in the town of Amesbury were exposed to the nerve agent Novichok, the same material used to poison a former Russian spy in a nearby area last spring.
Medical officials said Wednesday that people who had been in the area where the couple had been should take precautions and wash their clothes.
Police said it is unclear if this incident is linked to the earlier poisoning of Sergei and Yulia Skripal, or if the batch was the same as the one that poisoned them on March 4.
Counter-terrorism police detective Neil Basu said it is unclear if the couple were targeted or if the poisoning was accidental.
Wiltshire police declared the case a "major incident" Wednesday, four days after the man and woman were found collapsed at a residential building in Amesbury, eight miles from Salisbury, where the Skripals were poisoned. A major incident designation allows British authorities to mobilize more than one emergency agency.
Residents felt a grim sense of deja vu.
"With the Russian attack happening not long ago, we just assumed the worst," said student Chloe Edwards, who said police and fire engines descended on a quiet street of newly built homes in Amesbury.
Edwards said she saw people in green suits — like those worn by forensics officers — and her family was told to stay indoors for several hours.
Police said officers initially were called Saturday morning about a collapsed woman, then were summoned back in the evening after a man fell ill at the same property. Police at first thought the couple, identified by friends as 44-year-old Dawn Sturgess and Charlie Rowley, 45, had taken a contaminated batch of heroin or crack.
Prime Minister Theresa May's office said she was being kept updated on the case, "which understandably is being treated with the utmost seriousness."
The emergency services' response echoes that in the case of Sergei Skripal, 67. The former Russian intelligence officer was convicted of spying for Britain before coming to the U.K. as part of a 2010 prisoner swap.
He had been living in Salisbury, a cathedral city 90 miles southwest of London, when he was struck down along with his 33-year-old daughter, Yulia, who was visiting him.
The Skripals' illness initially baffled doctors after they were found unconscious on a park bench in Salisbury. Scientists at Porton Down concluded they had been poisoned with Novichok, a type of nerve agent developed by the Soviet Union during the Cold War.
After spending weeks in critical condition, the Skripals were released from the hospital and taken to an undisclosed location for their protection. Doctors say they don't know what their long-term prognosis is.
Britain has accused Russia of poisoning the Skripals, a claim Moscow strongly denies. The case sparked a diplomatic crisis between Russia and the West, including the expulsion of hundreds of diplomats from both sides.