U.S. investigators are looking at a range of theories, including the possibility of a "viral" attack, to explain what may have sickened some American diplomats who were stationed in Havana, the State Department said on Tuesday.
U.S. experts have yet to determine who or what was behind the mysterious illnesses that began occurring in late 2016 and heightened tensions between the old Cold War foes. They have seen no evidence it was "an episode of mass hysteria" among the 24 affected U.S. personnel and family members, a senior State Department medical officer told a Senate hearing.
State Department officials testified that it was "incomprehensible" Cuba's Communist government would not have been aware of what happened or who was responsible, though they stopped short of assigning direct blame to Havana.
Cuban officials, who are conducting their own investigation, have denied any involvement or any knowledge of what was behind it, and have raised doubts that any kind of attacks took place against U.S. diplomats.
The administration of President Donald Trump, which has partly rolled back a detente with Cuba, responded last year by sharply drawing down U.S. embassy staff in Havana and in October expelled 15 Cuban diplomats.