London (CNN) — British counter-terrorism officers have joined the investigation into how two critically ill people came to be exposed to an "unknown substance" in a town in southern England.
The victims, a man and woman in their 40s, were found unconscious on Saturday in Amesbury, just a few miles from Salisbury, where former double agent Sergei Skripal and his daughter were poisoned with a military-grade nerve agent in March. The UK government suspects the Skripals were victims of a Russian attack.
In a statement, London's Metropolitan Police said: "As you would expect, given the recent events in Salisbury, officers from the counter terrorism network are working jointly with colleagues from Wiltshire Police regarding the incident in Amesbury. As Wiltshire Police have stated, they are keeping an open mind as to the circumstances surrounding the incident and will update the public as soon and as regularly as possible."
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The two victims are currently in critical condition, according to police in Wiltshire, the county in which Amesbury is located.
They were initially believed to have fallen ill after using crack cocaine or heroin, but are now "receiving treatment for suspected exposure to an unknown substance," Wiltshire Police said in a statement.
"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 statement said.
British Prime Minister Theresa May and government ministers were treating the incident with "the utmost seriousness," her spokesman told CNN.
Church, pharmacy under police guard
On Tuesday, Amesbury Baptist Church was cordoned off, with police standing guard at the entrance. Church secretary Ray Collins told CNN he believed the victims attended a family fun day at the church on Saturday, though he did not know who the pair were.
A police officer stands outside the Amesbury Baptist Church in Wiltshire.
A police officer stands outside the Amesbury Baptist Church in Wiltshire.
A Boots pharmacy in the town center was also cordoned off, with police at the entrance telling CNN it was closed "as a precaution."
Salisbury District Hospital is open as usual and is advising people still to attend routine appointments unless they are contacted to state otherwise.
A Public Health England (PHE) spokesman said: "The current advice from PHE England, based upon the number of casualties affected, is that it is not believed that there is a significant health risk to the wider public. This will be continually assessed as further information becomes known."
The March incident in Salisbury sparked a diplomatic row between the UK and Russia after British investigators blamed Moscow for the attack.
Russia has denied the allegations, but more than 20 other countries kicked Russian diplomats out of their countries in a show of support for the UK.
Sergei and Yulia Skripal have since been discharged from the hospital.