UK officials 'know more' about what may have poisoned Russian spy

"If evidence emerges of state responsibility, the government will respond appropriately and robustly," he said.

Skripal, 66, and his daughter, Yulia, 33, are critically ill in a UK hospital after passing out on a shopping center bench in the English city of Salisbury, having suffered "suspected exposure to an unknown substance," police said.

Skripal was convicted in Russia of spying for Britain before being granted refuge in the UK after a high-profile spy swap in 2010 between the United States and Russia.

He is believed to have lived in the UK since his release from Russian custody in 2010. His daughter was visiting from Russia.

She is thought to be one of the few members of his immediate family still alive after his wife, Lyudmila, and son, Alexander, died in recent years.

Johnson's comments Tuesday drew an acerbic response from Russia's embassy in London, which released a statement saying it "looks like the script of yet another anti-Russian campaign has already been written."

In a statement, the embassy said it was "impressed" by Johnson's comments, saying he spoke "in such a manner as if the investigation was already over and Russia was found responsible for what had happened in Salisbury."

"We regret that instead of a proper official clarification on the issue the foreign secretary chose to threaten Russia with retribution," it read.

Russia had not received an official request from British authorities to assist in the investigation, Russian Foreign Ministry spokeswoman Maria Zakharova told reporters Wednesday during a briefing.

Johnson also called into question the UK's involvement in the 2018 World Cup, which kicks off in Russia in 100 days. A UK Foreign Office representative later clarified those comments, stating that Johnson was referring to the UK's diplomatic representation at the event, rather than the English soccer team, which has qualified for the tournament.

Salisbury, perhaps best known as the stopping-off point for tourists visiting nearby Stonehenge, has become the unlikely center of an extensive police probe into the mysterious circumstances surrounding how Skripal and his daughter fell ill.

Authorities have declined to name the substance to which the pair are suspected to have been exposed.

Investigators wearing protective gear and masks were seen Tuesday evening inside Zizzi's restaurant near Salisbury city center, where the Skripals were found. It's one of a number of sites that are being examined in the area. Police are keen to speak to anyone who visited the restaurant or the nearby Bishop's Mill pub on Sunday afternoon.

A small number of emergency services personnel were treated immediately after attending a scene of suspected contamination, local police said Tuesday. One of them remains in the hospital.

Additional cordons have also been placed at a scene in the nearby town of Amesbury, police said Wednesday in a statement. This is linked to the investigation and is a precautionary measure, they said. The scene, near Solstice Park, is believed to be an ambulance base.

Local convenience store manager Ebru Ozturk told CNN that Skripal was a "kind customer" who would usually come in once a week and buy Polish smoked bacon and scratch cards.

"His wife died a few years ago. He was feeling bit sad. He started to get used to living on his own after the wife died," said the manager of the Bargain Stop convenience store in Salisbury.

"He is (a) regular customer, he is so kind and he seems to me an educated person. Very polite," said Ozturk. "I don't talk too much to the customers, but he was, you know, one of the very kind customers."

CCTV footage showed Skripal talking to Ozturk and buying items at the store on February 27, five days before he was apparently poisoned.

Ozturk said Skripal never mentioned having a son and never mentioned his past, though he did say he had a daughter, whom Ozturk had never met.

Johnson said Tuesday that there were "echoes" in this case of what happened to former Russian spy Alexander Litvinenko, who died a slow death after drinking tea laced with highly radioactive polonium-210 in 2006 in a hotel in the Mayfair section of London.

A detailed UK inquiry later concluded that Russian President Vladimir Putin probably approved the operation by Russian agents to kill Litvinenko. At the time, the Russian Foreign Ministry dismissed the UK investigation as politically motivated.

Yvette Cooper, an opposition Labour MP and chairwoman of the parliamentary Home Affairs Select Committee on Tuesday called for a review of all suspicious deaths in the UK that could be linked to the Russian state.

In a letter to the home secretary, Cooper referred to media reports that suggested 14 deaths of Russians in the UK had raised concerns.

"Surely a review of all these cases and the decisions made on them, in light of this further reported evidence, carried out by the NCA, would be a prudent course of action to ensure that -- as the foreign secretary said today -- no attempt on an innocent life on British soil should go uninvestigated or unpunished," Cooper said.

Skripal arrived in the UK as part of an elaborately choreographed spy swap conducted by the United States and Russia in which the two countries exchanged agents on chartered planes on the runway at an airport in Vienna, Austria.

Skripal had been sentenced in 2006 to 13 years in prison for spying for the UK, according to previous reports by Russia's state news agency RIA Novosti.

State news quoted Russia's Federal Security Service, or FSB, saying Skripal, a former Russian military intelligence colonel, was convicted for "treason in the form of espionage" and that he had been recruited by Britain's MI6 intelligence service.

The report said he had spied for Britain as an intelligence officer during the 1990s and had continued to communicate with MI6 after his retirement in 1999.

Russian court officials said Skripal had received at least $100,000 for his collaboration with MI6, RIA Novosti reported.