Time and again, foes of Mr. Putin’s have died suddenly in Britain, under suspicious circumstances. In the most notorious case, Alexander V. Litvinenko, a former Russian agent who was harshly critical of Mr. Putin, was fatally poisoned in 2006 with a rare radioactive metal, and an inquiry later concluded that he was assassinated by Russian operatives, probably with Mr. Putin’s approval.

“I’m sure that in some of these cases, there is a relatively natural explanation, but it is beyond the bounds of probability that they all are,” said James Nixey, manager of the Russia and Eurasia program at Chatham House, a British foreign affairs think tank.

The British government has been accused of being less than eager to get to the bottom of those deaths, or to hold anyone responsible, but political and security analysts say this time is likely to be different. Given the government’s sensitivity to that criticism, and the intense worldwide attention on the Skripal case, a thorough investigation is probably unavoidable, they say, and if Russian involvement is found, an aggressive response may be inevitable, too.

The resources and expertise involved in producing and using a nerve agent suggest the involvement of a military or intelligence agency, as in two highly publicized episodes last year: Syrian government forces used sarin gas, a nerve agent, against a rebel-held village, and the North Korean government is believed to have been behind the assassination of the half brother of the country’s leader using another nerve agent, VX.

“We can’t say for sure right now, but the more sophisticated and the rarer the poison, the more likely it is to come from the Russian state or elements within it,” said Ben Judah, a biographer of Mr. Putin who has also researched the lives of Russian expatriates in Britain.