[Breaking news update 5:18 p.m. ET]
London Metropolitan Police say two people found unconscious in southern England were exposed to Novichok, the same nerve agent used to attack an ex-Russian spy in March.
The pair remain in critical condition in hospital, Neil Basu, Assistant Commissioner for Counter Terrorism at London Metropolitan Police, said Tuesday.
[Last update published 1:47 p.m. ET]
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 were hospitalized 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.
British news agencies have named the two people as Charlie Rowley, 45, and Dawn Sturgess, 44. Sam Hobson, a friend of the pair, also confirmed their names in an interview with Agence France-Presse.
"Paramedics from the South Western Ambulance Service were called to an address in Muggleton Road, Amesbury, on the morning of Saturday, 30 June, after a woman collapsed at the property," Deputy Chief Constable of Wiltshire Police Paul Mills told reporters on Wednesday afternoon. "They were called back later the same day, supported by colleagues from Dorset and Wiltshire Fire and Rescue Service, following reports a man had also fallen unwell in the same location."
He added it was not yet clear if a crime has been committed, but that inquiries are ongoing and that police are keeping an "open mind."
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."
The two victims -- both British nationals local to the area -- 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 from a contaminated batch, but are now "receiving treatment for suspected exposure to an unknown substance," Wiltshire Police said.
"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 Boots pharmacy in the town center was also cordoned off, with police at the entrance telling CNN it was closed "as a precaution."
Additionally, the Queen Elizabeth Gardens and a nearby supported-living facility in Salisbury have been closed off, as has another property in Amesbury, according to Wilshire Police, who said the scenes have been shuttered "as a precautionary measure."
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.
Sergei and Yulia Skripal have since been discharged from the hospital.