He wrote that two senior officials — Brian P. McKeon, a deputy secretary of state, and Ambassador Pamela Spartan — were leading a State Department task force that has begun to hold town-hall-style meetings in embassies that were suspected targets. There has been a similar effort underway at the C.I.A., where Mr. Burns, who met with victims on his first day on the job, recently told NPR that he thought about 100 officers and their families had been afflicted.
It “is real and it’s serious,” Mr. Burns said in the interview. He said only a few powers had the technology and the reach to conduct attacks around the globe — since the initial reports in Cuba, they have ranged from Europe to China to Washington — but Mr. Burns hesitated to say that the government of Vladimir V. Putin was responsible.
“Could be, but I honestly cannot — I don’t want to suggest until we can draw some more definitive conclusions who it might be,” Mr. Burns said. “But there are a number of possibilities.”
Investigations have attributed some of the reports to other causes. This spring, for example, American military personnel operating in Syria suspected that a sudden illness may have been caused by a Russian aircraft that could have directed microwaves at them; it was later determined they had food poisoning. But studies of victims elsewhere concluded that there was evidence of traumatic brain injury, though without the kind of impact normally associated with concussions.
Throughout the Cold War, Vienna was central to East-West diplomacy. Officials say they do not think recent reports of attacks there, which are still unconfirmed, are related to that, though the city has been a site of negotiations over resuming the Iran nuclear deal.
Some officials suspect that along with Russia, Iran may be responsible for some attacks, but there is also a focus on Cuba, China and other nations. The mystery has been compounded by the fact that some attacks took place on Russian and Chinese soil, including against a C.I.A. officer in Moscow four years ago and a number of State Department officials in Guangzhou, China, in 2018.
Speaking for the White House, a senior administration official said the National Security Council was “leveraging a broad array of scientific and medical expertise from within the government and outside of it to explore multiple hypotheses and generate new insights” into the episodes to “protect our personnel and identify who or what is responsible.”
One element of that effort, officials say, is to develop portable sensors that could be widely distributed to detect attacks. But it is hard to ensure that the sensors will work, one official said, without any certainty that microwaves are the cause of the unexplained illnesses. And even if they are the cause, the sensors would have to be able to pick up signals across a large part of the electromagnetic spectrum.
Julian Barnes contributed reporting.