That hypothesis is plausible, but so are others, said Dan Kaszeta, a former chemical and biological weapons adviser to the White House and the United States Secret Service. “There’s too many variables here,” he said.
Russia categorically denies involvement in either case, said Dmitry S. Peskov, a spokesman for President Vladimir V. Putin. “We are, of course, concerned after all by the repeated use of such substances in Europe,” he said in a conference call with reporters. “Although, on the other hand, we do not have information about what kinds of substances were used, how they were used.”
Russian officials quickly offered alternative theories, as they did after the Skripal poisoning, including one that says the British could have planted the nerve agent. Sergei Zheleznyak, a deputy speaker of the Russian Parliament, suggested that the British authorities had concocted the case to sully Russia’s image while the country hosts the World Cup.
“A huge number of British fans, despite the warnings from their government, came to support their team,” Mr. Zheleznyuak told state television. “Their impressions are just destroying everything British propaganda built over the past few years. To break up this flow of really positive emotions that the British fans are sharing, they had to put something like this in the mass media.”
Nikolai Kovalyov, a former director of the Federal Security Service, the successor to the K.G.B., suggested a rogue scientist from the Porton Down laboratory, near Salisbury, was conducting experiments on people living nearby.
The Russian Embassy in the Netherlands wrote on Twitter, “How dumb they think” Russia is to stage the latest attack while Russia is hosting the soccer tournament, a publicity boon.
On Thursday, Sajid Javid, the British home secretary, said that the nerve agent that poisoned the couple in Amesbury was the same one used to target the Skripals, but he did not say whether it was from the same batch of Novichok.