AMESBURY, England (AP) — Four months after a quiet corner of England was plunged into a Cold War-style saga of spies, chemical weapons and international tensions, residents wondered today whether it was happening all over again.

Counterterrorism detectives and local police were investigating after a couple in their 40s fell critically ill from exposure to an unknown substance a few miles from where a former Russian spy and his daughter were poisoned with a nerve agent in March.

Scientists were working to identify the substance amid speculation the victims could have been sickened by residue from the poison that nearly killed Sergei and Yulia Skripal.

Wiltshire Police declared the case a “major incident” today, 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 mobilise 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 on Saturday evening.

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 were initially 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.

“However, further testing is now ongoing to establish the substance which led to these patients becoming ill,” said Deputy Chief Constable Paul Mills. “At this stage it is not yet clear if a crime has been committed.”

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

British media reported that samples of the mystery substance had been sent to the Porton Down defense research laboratory for testing.

Prime Minister Theresa May’s office said she was being kept updated on the case, “which understandably is being treated with the utmost seriousness.”