LONDON - British counterterrorism police said Tuesday that they are aiding the investigation into what caused a former Russian double agent to collapse, leaving him critically ill in a Salisbury hospital.

The spy, identified by the British media as 66-year-old Sergei Skripal, was found slumped on a bench in a shopping center on Sunday in the cathedral city of Salisbury. He was sitting next to a 33-year-old woman who is also in critical condition. The BBC identified the woman as Skripal's daughter, Yulia.

Read More: Why Europe and Canada may retaliate against bourbon, Harleys and Levi jeans

Police said they were being treated for "suspected exposure to an unknown substance."

"It's a very unusual case, and the critical thing is to get the bottom of its causes as quickly as possible," said Mark Rowley, head of counterterrorism policing in the United Kingdom. He told the BBC that the case would become a counterterrorism investigation "if necessary."

"We're doing all the things you would expect us to do. 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," he said.

Russian President Vladimir Putin's spokesman, Dmitry Peskov, told reporters Tuesday that the Kremlin knew nothing at all about the case and that it was ready to cooperate in the investigation if asked.

"We know that this tragic situation has happened, yet we have no information about its probable causes, what this man has been doing, and what this is about," Peskov said. He described accusations against Russia in the matter as predictable and "not long in coming."

The case drew comparisons to the 2006 poisoning of Alexander Litvinenko, who died in a London hospital bed three weeks after drinking tea laced with a radioactive substance. In 2016, a British public inquiry found that Putin had "probably" ordered the killing of Litvinenko, who was an outspoken critic of the Kremlin and a former KGB operative.

Litvinenko's wife, Marina, told the Times of London that when she heard of the suspected poisoning of Skripal and saw television footage of police in Salisbury wearing hazardous materials suits "it was kind of déjà vu."

Skripal was jailed in 2006 for passing the names of Russian intelligence agents to MI6, Britain's foreign intelligence service. He was released in 2010 under a high-profile spy swap that included four prisoners released by Moscow in exchange for 10 Russian spies living in the United States, including Anna Chapman. Skripal took refuge in the United Kingdom.

Eyewitnesses who saw Skripal and the woman on the bench Sunday said they didn't look well.

It seemed as if they had taken "something quite strong," Freya Church told the BBC. "On the bench there was a couple, an older guy and a younger girl. She was sort of leaned in on him. It looked like she had passed out, maybe. He was doing some strange hand movements, looking up to the sky."

---

Anton Troianovski in Moscow contributed to this report.