A woman found unconscious on a bench next to former Russian spy Sergei Skripal is Skripal's daughter, the BBC reports.
Both Yulia Skripal, 33, and her 66-year-old father, Sergei, are in a critical condition in hospital after being found unconscious on a bench near a shopping center in Salisbury in southern England on March 4.
British police are trying to establish what harmed the pair.
Several emergency services workers were assessed immediately after the incident -- and one remains in hospital, authorities say.
Skripal is a former colonel in Russia's GRU military intelligence agency who was convicted of passing state secrets to Britain in 2006 but was released from prison -- and sent to the West -- in a spy swap in 2010.
The unexplained incident swiftly drew comparisons with the death of Aleksandr Litvinenko, a former Russian security agent who fell ill and died in London in November 2006 after ingesting radioactive polonium-210.
A British inquiry concluded that the Russian government was behind Litvinenko's death and that Russian President Vladimir Putin "probably approved" the killing.
Russia insists it has "no information" on what could have led to the incident.
But Putin's spokesman called it a "tragic situation" and indicated that Moscow is ready to cooperate with British authorities if asked.
"Moscow is always open to interaction," Kremlin spokesman Dmitry Peskov said on March 6 when asked whether Russia would cooperate. He said that Russia has not received such a request.
While Skripal appeared to be one of several Russians who have suddenly become ill or died in Britain in recent years, British authorities said that nobody should assume his illness was the result of foul play.
The authorities "are conducting some extensive inquiries to determine exactly what led to these two people falling unconscious and clarify whether or not any criminal activity has happened," said Wiltshire police official Craig Holden.
Mark Rowley, Britain's top counterterrorism officer said that police investigating the incident are "alive to the fact of state threats."
"We're speaking to witnesses. We're taking forensic samples at the scene. We're doing toxicology work and that will help us to get to an answer," Rowley said.
Parts of the Salisbury center remained sealed off early on March 6 as emergency responders in hazardous material suits continued to canvas there.
Police vehicles were also seen at what was listed as Skripal's home in Salisbury.
The National Health Service said it had only limited information about the patients, but there "doesn't appear to be any further immediate risk to public health."
The substance has not been identified. But local media reported that emergency services suspect the powerful synthetic opiate fentanyl may have been involved.
"They looked like they'd been taking something quite strong," the BBC quoted an eyewitness as saying.
"She was sort of leant in on him. It looked like she had passed out maybe," the eyewitness said. "He was doing some strange hand movements, looking up to the sky."
Skripal was arrested in Moscow in December 2004 and convicted by a Moscow military court in August 2006 of "high treason in the form of espionage."
He was found guilty of passing the identities of Russian intelligence agents working undercover in Europe to Britain’s Secret Intelligence Service, or MI6, in return for $100,000.
Russia’s Federal Security Service (FSB) alleged he had begun working for MI6 while serving in the army in the 1990s.
The death of Litvinenko, who moved to Britain and had become a vocal critic of Putin, severely strained relations between London and Moscow.
Coming weeks after investigative journalist Anna Politkovskaya was shot dead in Moscow, it deepened concerns about the risks run by Russians who challenge the Kremlin -- wherever they live.
In findings issued in January 2016, British investigators said there was a "strong probability" that Litvinenko's poisoning was carried out by Russians Andrei Lugovoi and Dmitry Kovtun, acting under orders from the Federal Security Service (FSB).
They concluded that Litvinenko ingested polonium-210 while drinking tea in a luxury London hotel with Lugovoi and Kovtun. He died in a London hospital three weeks later, on November 23, 2006.
Russia has dismissed the inquiry as "opaque" and "politically motivated" and has refused to extradite the suspects.
Kovtun and Lugovoi, who is now a deputy in the Russian parliament, have denied involvement despite traces of polonium that British investigators say the two left across London.
Igor Sutyagin, a Russian arms control researcher who also part of the exchange of spies in 2010 and lives in Britain, told the online news organization RTVI that he only knew Skripal briefly, during the time they were being flown from Moscow to London.
He said the incident might have been insidious but appeared to downplay that possibility, suggesting that Russia might not want to risk a further blow to its reputation in the West.
"Such events are not really in Russia's interests right now," Sutyagin said. "The Kremlin has so many problems falling on its head these days, one more would be too much."
But in a reference to the killing of opposition politician Boris Nemtsov, who was shot dead near the Kremlin in Febuary 2015, he said that "Nemtsov set the pattern and now [the Russian authorities] have this reputation.”
With reporting by the BBC, Reuters, AP, AFP, and Press Association