Secretary of State Antony Blinken said on Thursday that US intelligence agencies have yet to determine the cause of mysterious symptoms affecting US diplomats around the world, nicknamed “Havana Syndrome”.
Mr Blinken made the comments during an interview with MSNBC after it was reported that more Americans in Geneva, Switzerland, and Paris, France are experiencing the symptoms, resulting in at least one returning to the US.
The unexplained phenomenon has now spread to roughly 200 people, and is no longer clustered in Havana, which drew the suspicions of US lawmakers and media figures.
“To date, we don’t know exactly what’s happened and we don’t know exactly who is responsible,” Mr Blinken told MSNBC on Thursday.
He went on to say that the admission was not an indication that the federal government was not taking the symptoms seriously, and said that his department was working “overtime” to find answers.
"There is no doubt in my mind that people have been directly and powerfully affected," he said.
"We are working overtime across the entire government to get to the bottom of what happened, who's responsible. And in the meantime to make sure that we're caring for anyone who's been affected and to protect all of our people to the best of our ability,” added the secretary.
The comments come after questions have been building for months about the strange, unexplained illnesses and a host of theories have been suggested, including the supposed existence of microwave-based weaponry.
Among the effects of the phenomenon are an increased feeling of pressure in one’s head, problems sleeping, dizziness, headaches, and other neurological issues that many said developed after they heard a loud, piercing sound. Experts on WebMD say the symptoms generally present as if one has suffered a severe head injury.
Mark Zaid, a lawyer for a number of those affected with the disease, told WebMD that the symptoms first originated in Americans not stationed in or around Cuba.
"It started long before Havana," Mr Zaid told the website. "It was just those incidents that brought it to light."
In November, the Biden administration named a new coordinator at the State Department to oversee a department-wide task force investigating the issue.
The appointment of the official, Jonathan Moore, came a month after a group of senators including members of both parties wrote to the White House to complain that the issue was not being treated with sufficient urgency.
"We ask that you take this step now to demonstrate that the State Department does take this matter seriously, and is coordinating an appropriate agency-level response,” wrote the senators.
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