Two U.K. citizens are critically ill in hospital after being exposed to the Novichok nerve agent — the same potentially lethal toxin that was used to poison former Russian spy Sergei Skripal and his daughter Yulia in March.

The man and woman, both British and named in U.K. media reports as Charlie Rowley, 45, and Dawn Sturgess, 44, were found unconscious on Saturday at a house in the small town of Amesbury, just nine miles from Salisbury in southern England where the Skripals were attacked.

They are now seriously ill in hospital; police said Thursday that they had been exposed to Novichok, a nerve agent developed in the Soviet Union in the 1970s.

The incident has brought up barely settled tensions between the U.K. and Russia. British Security Minister Ben Wallace has called on Russia to provide more details about the Skripal attack so that authorities can “keep people safe,” while one senior Russian lawmaker called on the U.K. to ask Russian experts to take part in the latest investigation.

As with the Skripal attack, Russia has denied any involvement in the latest incident.

The Russian embassy in the Netherlands tweeted that Britain was “dumb” if it thinks Moscow would stage a nerve agent attack during the World Cup soccer tournament, which it is hosting currently. Another Russian lawmaker Sergei Zheleznyak said the latest incident looked like an attempt to spoil England soccer fans’ positive perception of the tournament, Reuters reported.

The U.K. has not accused Russia of being behind the latest incident, with the working assumption being that the couple had a chance encounter with the nerve agent that might not have been properly disposed of after the Skripal attack.

However, Metropolitan Police Assistant Commissioner Neil Basu could not confirm Wednesday whether the nerve agent that the British man and woman had come into contact with was the same as that used on the Skripals.

Counter-terrorism police have been brought in to investigate how the victims were exposed to the nerve agent, which can be up to eight times as lethal as VX, the nerve agent used to kill North Korean leader Kim Jong Un’s half-brother last year.

Home Secretary Sajid Javid is scheduled to hold a meeting of the government’s emergency Cobra committee Thursday to discuss developments.

After weeks in a critical condition, the Skripals eventually recovered. Nonetheless, the Skripal attack caused deep damage to relations between the U.K. and Russia; the British government said Moscow was “highly likely” to have been behind the attack, given the high level of skill needed to produce Novichok.

Michelle Carlin, a toxicologist at Northumbria University in England, said the latest incident is surprising.

“It is difficult to speculate whether this is from the original source used in the Skripal case,” she said in a statement. “If it was from the same source, it is unusual that it has taken four months for someone to be affected by it.

“There are many factors that we are unaware of at this moment, but based on the fact that the agent has been identified quickly and the hospital has successfully treated the Skripals, hopefully the couple will recover well.”

After a second nerve agent poisoning in a quiet part of southern England, the country’s chief medical officer sought to reassure the British public that the risk was low.

The U.K. responded to the Skripals’ poisoning by expelling 23 Russian diplomats; a raft of its international allies followed suit in a show of solidarity. Russia has always denied any involvement in the poisoning, saying the attack was fabricated and an attempt by the U.K. to spread anti-Russian propaganda.