Wiltshire Police in the southern English town of Amesbury declared a "major incident" late on Tuesday night after two people were found unconscious after authorities suspect they came into contact with an "unknown substance."
The Wiltshire Police initially thought that the couple had been exposed to "heroin or crack cocaine from a contaminated batch of drugs," but are now treating them "for suspected exposure to an unknown substance."
The incident happened just a few miles away from Salisbury, where British police say Sergei Skripal, a Russian double agent living in the United Kingdom, and his daughter were poisoned with a military-grade nerve agent in March.
"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," the Wiltshire Police added, noting that both patients are listed in critical condition.
Russia was accused of being behind the incident in March and received widespread condemnation for the alleged attack.
Russia's U.N. Ambassador Vassily Nebenzia, fired back at the U.K. over the allegations, saying, "we have told our British colleagues that they are playing with fire and they will be sorry."