LONDON — The police scoured the area around Salisbury, England, for a container of a deadly chemical weapon on Monday, as high-ranking British officials suggested for the first time that Russia was probably responsible for a second set of nerve agent poisonings in the region.
British officials have said that a couple who were sickened this month in the Salisbury area, one of whom died on Sunday, had been poisoned with the same powerful nerve agent used in March, a few miles away, against a former Russian spy and his daughter.
But while government officials have accused the Kremlin of responsibility for the first poisonings, until Monday they refrained from assigning blame for the second, though they acknowledged a strong possibility that the two were related.
“The simple reality is that Russia has committed an attack on British soil which has seen the death of a British citizen,” the defense minister, Gavin Williamson, said in the House of Commons. “That is something that I think the world will unite with us in actually condemning.”
Sajid Javid, the home secretary, who has acted as the government’s primary spokesman on the matter, was more cautious in addressing Parliament hours later. The investigation is still underway, he said, and the government will not jump to conclusions.
But when asked directly if Russia was responsible for the latest poisonings, he said it was hard to see any “other plausible explanation.”
Dawn Sturgess, 44, and her boyfriend, Charlie Rowley, 45, became ill on June 30 at Mr. Rowley’s home in Amesbury, a town near Salisbury, and were hospitalized. Ms. Sturgess, a mother of three who lived in Salisbury, died on Sunday, and the case is now a murder investigation. They had been exposed to an agent known as Novichok.
President Vladimir V. Putin’s government has strenuously denied any involvement in either case, floating an array of theories about what might have happened and nominating an assortment of possible culprits.
The defense secretary’s accusation came a few hours after Britain’s top counterterrorism police officer, Neil Basu, made a sobering admission: Even after repeatedly searching the homes of the two recent victims and places they were thought to have been shortly before they took ill, hundreds of law enforcement officers and others combing the Salisbury area still have not found the object that poisoned them.
“Our focus and priority at this time is to identify and locate any container that we believe may be the source of the contamination,” Mr. Basu, assistant commissioner of the Metropolitan Police, told reporters outside New Scotland Yard.
That means it remains possible that someone else could be poisoned. “I simply cannot offer any guarantees,” he said, reiterating the government’s advice that people not pick up any unknown objects.
“Their reaction was so severe, it resulted in Dawn’s death and Charlie being critically ill,” Mr. Basu said. “This means that they must have got a high dose, and our hypothesis is that they must have handled a container we are now seeking.”
Chemical weapons experts say it was likely that whoever carried out the earlier attack transported the nerve agent in a sealed container, and discarded the container at another location, where it was found by Mr. Rowley or Ms. Sturgess. They say the poison could remain potent for months, particularly in a closed container.
Mr. Javid said, “the nerve agent that’s been used in this incident is the same as the March 4 incident, but we have not, the scientists have not been able to identify or determine if it is the same batch.”
On Monday, the authorities evacuated a bus in the city center and, for a few hours, established a wide security cordon around it, raising fears of another nerve agent incident. The police later said nothing out of the ordinary had been found.
A police officer was hospitalized out of fear that he might be showing symptoms of poisoning, but that, too, was a false alarm, and he was released.
The March attack sickened Sergei V. Skripal, a former Russian intelligence agent who had sold information to Britain and was living in Salisbury; his daughter, Yulia S. Skripal, who was visiting from Russia; and Detective Sgt. Nick Bailey of the Wiltshire Police, who was one of the first officers to look into the case. All three survived; officials have not said publicly whether they have any lasting impairments.
Laboratory testing showed the substance that poisoned them was Novichok, the British government has said, part of a class of nerve agents developed first in the Soviet Union and later in Russia. That conclusion was supported by further testing by the Organization for the Prohibition of Chemical Weapons, the international body that monitors compliance with a chemical weapons ban treaty.
The organization — at Britain’s behest and over Russia’s objections — recently decided that it will not only investigate whether chemical weapons have been used but will try to determine who used them.
The attack on the Skripals worsened the already tense relations between Russia and the West, prompting the expulsion of Russian diplomats and embassy workers from several countries and retaliatory expulsions by Moscow.
The British government has also promised to tighten controls on wealthy Russians who moved their assets in and out of Britain, a move seen as a blow to Mr. Putin’s powerful circle of allies, many of whom have ties to Britain.
British officials have said they will raise the Novichok attack at a meeting of the leaders of NATO countries.