The Metropolitan Police force said Thursday that “following further tests of samples from the patients, we now know that they were exposed to the nerve agent after handling a contaminated item”. Detectives have cordoned off several sites in Amesbury and Salisbury as they search for the source of the contamination.

Russia, which is currently hosting the soccer World Cup, has denied any involvement in the March incident and suggested the British security services had carried out that attack to stoke anti-Moscow hysteria.

UK couple was poisoned with Novichok nerve agent, police say

“The eyes of the world are currently on Russia, not least because of the World Cup,” British Home Secretary Sajid Javid said. “It is now time that the Russian state comes forward and explains what has gone on.

“It is completely unacceptable for our people to be either deliberate or accidental targets, or for our streets, our parks and towns to be dumping grounds for poison,” he told parliament.

UK Prime Minister Theresa May also spoke out on Thursday. “To see two more people exposed to the Novichok in the UK is obviously deeply disturbing, and the police I know will be leaving no stone unturned in their investigation,” she said during a visit to German Chancellor Angela Merkel in Berlin.

Germany was one of a number of British allies to expel Russian diplomats after London blamed Moscow for the Skripal poisoning, prompting May to again thank Merkel Thursday for her country’s “unwavering support”. But UK ministers do not so far believe the latest incident was a deliberate attack.

Novichok is a military-grade nerve agent developed by the Soviet Union during the cold war.

After coming into contact with the poison, Skripal, 67, and his 33-year-old daughter Yulia were treated for weeks before being released from hospital. The police said they suspected the nerve agent may have been smeared on a front door handle in liquid form.

“The Russian state could put this ‘wrong’ right. They could tell us what happened, what they did and fill in some of the significant gaps that we are trying to pursue,” British Security Minister Ben Wallace said. “I’m waiting for the phone call from the Russian state.”

Russia, which is currently hosting the soccer World Cup, has denied any involvement in the March incident and suggested the British security services had carried out the attack to stoke anti-Moscow hysteria.

Sergei Skripal, the former Russian spy poisoned by nerve agent, discharged

“I am sure that for everything that Theresa May’s government has stirred up, this government and its representatives will have to apologise to Russia and the international community,” foreign ministry spokeswoman Maria Zakharova told reporters.

“As is tradition in Britain, it will happen later but it will happen.”

Kremlin spokesman Dmitry Peskov said he did not know who Ben Wallace was but said Russia had offered Britain its assistance in investigating the nerve agent attack and had been rebuffed.

In the latest twist in one of the most mysterious poisonings in recent years, the two Britons, who were taken ill on Saturday, were initially thought to have taken an overdose of heroin or crack cocaine.

But tests by the Porton Down military research centre showed they had been exposed to Novichok. Britain has notified the global chemical weapons watchdog, the Organisation for the Prohibition of Chemical Weapons (OPCW), about the poisoning.

It is unclear how the two Britons, whose background has nothing to suggest a link to the world of espionage or the former Soviet Union, came into contact with the poison, which is slow to decompose.

“The working assumption would be that these are victims of either the consequences of the previous attack or something else, but not that they were directly targeted,” Wallace said.

Paramedics were called on Saturday morning to a house in Amesbury after the woman, named by media as Dawn Sturgess, collapsed. They returned later in the day when the man, Charlie Rowley, also fell ill.

Health chiefs said the risk to the public was low, repeating their earlier advice that the public should wash their clothes and use cleansing wipes on personal items.

UK police: daughter of poisoned ex-spy Sergei Skripal turns down help from Russia

But the exposure of two British citizens to such a dangerous nerve agent will stoke fears that Novichok could be lingering at sites around the ancient English city of Salisbury.

Andrea Sella, professor of inorganic chemistry at University College London, said Novichok nerve agents were designed to be quite persistent and did not decompose quickly.

“That means that if a container or a surface was contaminated with this material it would remain a danger for a long time,” Sella said. “It will be vital to trace the movements of this couple to identify where they might have come into contact with the source.”

After the Skripal poisoning, police investigators in protective hazmat suits scoured Salisbury. They may return, police said.

Vladimir Putin dismisses G7 criticism of Russia as ‘babbling’

The March attack prompted the biggest Western expulsion of Russian diplomats since the cold war as allies sided with May’s view that Moscow was either responsible or had lost control of the nerve agent.

Moscow hit back by expelling Western diplomats, questioning how Britain could know that Russia was responsible and offering rival interpretations, including that it amounted to a plot by British secret services.

Additional reporting by Reuters