Authorities maintained a cordon near the spot a bench near a shopping mall where former double agent Sergei Skripal and an unidentified woman collapsed Sunday in Salisbury, 145 kilometers southwest of London.
British media reported that the woman was Skirpal's daughter.
Though authorities are trying to keep an open mind, the incident drew parallels to the death of former Russian agent Alexander Litvinenko, who was poisoned with radioactive polonium 11 years ago in London.
"But likewise we have to be alive to the fact of state threats as illustrated by the Litvinenko case."
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Wiltshire Police, which is responsible for the Salisbury area, said the man and woman appeared to know one another and had no visible injuries.
"They are currently being treated for suspected exposure to an unknown substance. Both are currently in a critical condition in intensive care," police said in a statement. The discovery led to a dramatic decontamination effort. Crews in billowing yellow moon suits worked into the night spraying down the street, and the Salisbury hospital's emergency room was closed.
"Police had a good look at the footage and were interested in these two people. It was the only image they took away," said Cain Prince, 28, the manager of a nearby gym. "They wanted a list of everyone in the gym between 3 p.m. and 4 p.m. as well."
Public records list Skripal as having an address in Salisbury.
Skripal served with Russia's military intelligence, often known by its Russian-language acronym GRU, and retired in 1999. He then worked at the Foreign Ministry until 2003 and later became involved in business.
At the time of Skripal's trial, the Russian media quoted the FSB domestic security agency as saying that the damage from his activities could be compared to harm inflicted by Oleg Penkovsky, a GRU colonel who spied for the United States and Britain. Penkovsky was executed in 1963.
The circumstances surrounding Sunday's incident were still murky and police urged the public not to speculate. But few could avoid invoking the name of Litvinenko the former Russian agent who died after drinking polonium-210-laced tea in a swanky London hotel in 2006.
A British judge wrote in a 2016 report that Litvinenko's death was an assassination carried out by Russia's security services with the likely approval of Putin. The Russian government has denied any responsibility.
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