“The idea that someone could put together some sort of action against them, 24 of them, and the Cuban government not know who did it, it’s just impossible,” Mr. Rubio said in a Senate hearing in January.
He and others also speculated that another foreign power, perhaps Russia, had been behind or part of the Havana attack. With similar problems appearing elsewhere in the world, that argument will probably gain steam, as the Cubans are unlikely to have the capacity to engage in similar attacks elsewhere in the world.
More than a year after the mystery attacks began in Cuba, there are few indications that American investigators are any closer to identifying what may have caused the problem.
About 2,000 employees of the United States government are posted in China, said Ms. Lee, the embassy spokeswoman. Diplomats based in the country are probably among the most closely watched by Chinese security services.
But it is unclear what might drive Chinese agents to risk inflicting brain injury on an American government employee, especially in a heavily commercial posting like Guangzhou, which is 1,200 miles south of Beijing. The United States Consulate there is housed in a secure, boxlike building that was opened in 2013.
The Global Times, a Beijing newspaper that often gives strident voice to the Chinese government’s views, said in an online editorial that the notion that China would attack foreign government personnel was “totally preposterous.”
“We’re very sure that the ‘brain injury’ to the American consulate employee can’t possibly have any ‘background,’” the editorial said. “There should be repeated screening for individual health causes, including psychological causes.”
Still, the State Department was not taking chances.
“While in China, if you experience any unusual acute auditory or sensory phenomena accompanied by unusual sounds or piercing noises, do not attempt to locate their source,” the notice from the consulate said. “Instead, move to a location where the sounds are not present.”