LONDON — The British government on Thursday called on Russia to explain “exactly what has gone on” after a British couple fell into a coma following exposure to the same type of Soviet-era nerve agent used in March to poison a former spy and his daughter.

The couple in the latest incident was exposed to the Novichok nerve agent after “handling a contaminated item,” the Metropolitan Police said in a statement Thursday evening.

Investigators are trying to determine whether this was residual contamination from the March attack, when Sergei Skripal and his adult daughter, Yulia, were poisoned just eight miles away. British authorities linked that attack to the Russian government. Moscow denied any involvement.

Britain’s security minister, Ben Wallace, told the BBC that the “working assumption” is that the British pair sickened in the town of Amesbury on Saturday was not directly targeted.

British officials say the pair may have come into contact with nerve agent at a location not covered in the Skripal decontamination sites. Health officials alternatively could have failed to eliminate exposure risks during their extensive cleanup.

Another, more troubling, scenario would be that the pair was exposed to a different batch of Novichok.

Witnesses told British media that the couple — identified as Charlie Rowley, 45, and Dawn Sturgess, 44 — were frothing at the mouth and incoherent before becoming comatose. They are in critical condition at Salisbury District Hospital, the same hospital where the Skripals received care.

“It is completely unacceptable for our people to be either deliberate or accidental targets,” Sajid Javid, Britain’s home secretary, told Parliament on Thursday, adding that it was also unacceptable for British streets or parks to be “dumping grounds for poison.”

Russia hit back, urging Britain to avoid playing politics.

Maria Zakharova, Russia’s Foreign Ministry spokeswoman, told reporters in Moscow that Britain should avoid “dirty political games” and should instead cooperate with Russian law enforcement.

The Russian Embassy in London had called for a joint investigation into the Salisbury incident.

The latest incident threatens to further damage British-Russian relations, which are already in a deep freeze after the Skirpal poisonings.

Britain condemned that attack as a hostile act by Russia and went on to expel 23 Russian diplomats. At least 26 other countries joined in retaliatory measures by also expelling Russian diplomats. British government officials and members of the royal family have also boycotted the World Cup in Russia.

“I know that many of you will question whether this incident is linked to that one,” Javid said to Parliament on Thursday. “That is clearly the main line of inquiry. However, we must not jump to conclusions, and we must give the police the space and the time to carry out their investigations.”

“It’s not looking like a new attack,” said Hamish de Bretton-Gordon, a former commander of the British army’s chemical and biological weapons unit. Although he is not directly involved with the investigation, he said that “from what I understand, this is debris or collateral from the original attack, possibly contained in a syringe or medical container.”

He said that an assailant could have discarded the Novichok residue four months ago in the park in Salisbury or in the nearby river, which is running low.

“Why the couple then picked it up . . . we don’t know. Probably just very bad luck,” he said.

Health officials have insisted that the risk to the wider public is low.

They have advised as a “highly precautionary measure” that those who visited the sites that Rowley and Sturgess went to on Friday and Saturday wash their clothes and wipe down personal belongings such as phones and handbags. They also urged locals not to pick up any unknown items.

Anton Troianovski in Moscow contributed to this report.