LONDON—It was already billed as one of the tensest summits in NATO’s 69-year history, and that strain has been dramatically increased by news that two more people have been struck down on British soil by the deadly Russian nerve agent Novichok.

A British couple is in critical condition at a hospital in Salisbury after coming into contact with the poison, which was likely left behind after the attempted assassination of Sergei and Yulia Skripal.

Scotland Yard’s head of counterterrorism announced that tests on the couple, who collapsed at home on Saturday, had confirmed fears that the nerve agent had struck again. It is not known how Dawn Sturgess, 44, and Charlie Rowley, 45, came across the substance, but police cordoned off a public park in Salisbury as they tried to locate the source.

The Skripals managed to survive the first known Novichok attack outside Russia, but relations between London and Moscow remain at a breaking point. Britain accused Russia of carrying out the attack, a claim the Russian ambassador ridiculed during a series of inflammatory press conferences in the heart of London.

President Trump initially appeared reluctant to side with Britain in pointing the finger at President Putin in March, but he eventually agreed that it looked as though Russia had ordered a hit on a former double agent.

The infection of two British citizens so long after the initial attack highlights just how reckless the assassination attempt was.

While Britain continues to boycott Russia’s World Cup—no British officials or dignitaries have attended the tournament—the White House announced last week that Trump and Putin would hold their first formal summit in Helsinki, Finland, just a few days after the NATO meeting.

NATO was established as a bulwark against Soviet aggression at the start of the Cold War, but Trump has continually groused about its existence. Last week, he reportedly complained that “NATO is as bad as NAFTA”—the trade agreement he has described as one of the worst deals of all time. The off-camera remark was reportedly made at the G7 summit, where he also called for Russia to be reinstated after its suspension for illegally annexing Crimea in 2014—the first land grab from a European country since World War II.

The need for toughened resolve against Russia was reinforced earlier this year when British scientists announced that a nerve agent created in Soviet chemical weapons labs was responsible for the collapse of the Skripals—who were found foaming at the mouth on a bench in the center of Salisbury.

Sergei Skripal survived the poisoning on March 4 and was released from hospital two months later, his daughter was discharged a few weeks earlier. Putin continued to deny that Russia had anything to do with the attack. “If a military-grade poison had been used, the man would have died on the spot,” he said. “Thank God he recovered.”

It is not yet known how Sturgess and Rowley came into contact with traces of the agent, but the closure of a section of local park will heighten fears that members of the public and children could have been exposed to the poison.

Neil Basu, who heads Britain’s counterterror command, said samples from the patients were transported to the Porton Down military lab on Monday after their symptoms worsened.

“Following the detailed analysis of these samples, we can confirm that the man and woman have been exposed to the nerve agent Novichok, which has been identified as the same nerve agent that contaminated both Yulia and Sergei Skripal,” he said.