British counterterrorism police say a former Russian double agent, whom Russian President Vladimir Putin had vowed to kill, was poisoned by a nerve agent.

The comments came after the British government’s high-level emergency committee known as COBRA was updated Wednesday on a probe into the poisoning of Sergei Skripal and his 33-year-old daughter.

“This is being treated as a major incident of involving attempted murder by the administration of a nerve agent,” assistant police commissioner Mark Rowley said. He said father and daughter remain critically ill. Rowley would not identify the exact substance used or how it was delivered.

Police also confirmed Wednesday that one officer who was part of the initial response to the incident was hospitalized in serious condition.

The poisoning Sunday of the two in the normally quiet town of Salisbury is threatening a full-scale security and diplomatic crisis for Britain, with lawmakers demanding the government launch an urgent inquiry into more than a dozen recent suspicious deaths in Britain, all potentially tied to Russian intelligence services.

Skripal, 66, and his daughter, Yulia, are fighting for their lives after being found unconscious on a bench outside a shopping mall. Police have been examining CCTV footage and reportedly have focused their attention on a man and woman spotted nearby. 

In this image taken from security camera footage f
In this image taken from security camera footage from PA shows a man and woman as they walk through an alleyway connecting a Zizzi's restaurant to a bench where former Russian double agent Sergei Skripal was found in Salisbury on Sunday March 4, 2018.

On Tuesday, Britain’s foreign minister, Boris Johnson, prompted sharp Russian rebuttals when he assured British lawmakers the government would get to the bottom of the mystery and threatened the imposition of new sanctions on Russia, if the Kremlin were found to have been responsible. Johnson said while he was not pointing the finger at this stage, he described Russia as “a malign and disruptive force.”

His remarks were characterized by Russian Foreign Ministry spokeswoman Maria Zakharova as “wild.” Russian diplomats in London accused Johnson of “demonizing” their country.  The incident is drawing comparisons to Alexander Litvinenko, a highly public critic of President Putin and a Russian KGB officer-turned-British intelligence agent, who died agonizingly, days after drinking tea laced with radioactive polonium-210 in a London hotel in 2006.  British doctors struggled in that case to identify the substance that killed him.

FILE - Alexander Litvinenko, former KGB spy and au
FILE - Alexander Litvinenko, former KGB spy and author of the book "Blowing Up Russia: Terror From Within" photographed at his home in London, May 10, 2002.

A British inquiry concluded the Russian leader probably approved the killing. The conclusion was dismissed angrily by the Kremlin as a politically motivated smear.

An eyewitness to the discovery of Skripal and his daughter, Jamie Paine, told British reporters the woman was passed out, frothing at the mouth and her eyes “were wide open, but completely white.” He said, “The man went stiff, his arms stopped moving, but he was still looking dead straight.” Adding to alarm, one of the emergency service workers who attended the pair has also been hospitalized.

Skripal, who served in Russia’s military intelligence agency, GRU, was exchanged in a Cold War-type spy swap in 2010 on the runway at Vienna’s airport. After serving four years in prison in Russia for spying for Britain’s espionage service, MI6, he was one of four Russian double agents exchanged for 10 Russian sleeper agents expelled from the United States, including Manhattan socialite and diplomat's daughter Anna Chapman.

At the time, Putin, a former KGB officer, issued televised threats against those who had betrayed Russia. “Traitors will kick the bucket.  Trust me. These people betrayed their friends, their brothers-in-arms. Whatever they got in exchange for it, those 30 pieces of silver they were given, they will choke on them,” he said.

In 2006, a new Russian law was adopted formally permitting extra-judicial killings abroad of people Russian authorities deemed extremist or terrorists, allowing the Russian president alone to order a killing.  

Skripal had spied for Britain during the 1990s and continued to communicate with MI6 after his retirement in 1999 from the GRU, while working at the Ministry for Foreign Affairs in Moscow.

FILE - A police tent covers the spot in Salisbury,
A police tent covers the spot in Salisbury, Britain, March 6, 2018, where former Russian double agent Sergei Skripal and his daughter Yulia were found critically ill Sunday following a suspected exposure to an 'unknown substance.'

Russian prosecutors said Skripal received at least $100,000 for his collaboration with MI6, according to Russian news outlets. At his trial he admitted selling the names, addresses and code names of “several dozen" Russian agents operating in Europe to MI6 over a period of 10 years. British intelligence officials say Skripal identified as many as 300 Russian spies and moles.

A year after the spy swap, he bought a house in Salisbury for $360,000. He lived apparently quietly there with his wife, Lyudmila, until her death from cancer five years ago. But a relative told BBC Russia, “From the first day, he knew it would end badly, and that he would not be left alone,” he said.

As British investigators piece together what happened to Skripal, senior British lawmakers say other suspicious Russia-linked deaths during the past two decades need to be re-examined.

Among them: former oligarch Boris Berezovsky; Scot Young, a businessman impaled on an iron fence after a fall from a window; Badri Patarkatsishvili, a Georgian oligarch who died of an apparent heart attack in 2008; Yuri Golubev, an outspoken Putin critic, and Alexander Perepilichny, who fled Russia for London and gave evidence of high-level corruption to Swiss authorities.

Britain’s foreign secretary, Boris Johnson, during a press conference late Wednesday held with the Saudi Crown Prince, who is visiting London, said:“If this does turn out to be in any way turn out to behostile activity by another government or led by another government then the people of this country can be absolutely sure the UK will respond robustly.”

On Thursday, Amber Rudd, Britain’s interior minister is due to make a statement in the House of Commons, and officials say she’s likely to announce an inquiry into the other suspicious recent Russia-linked deaths on British soil.