The U.S. would be "putting people intentionally in harm's way" if it sent diplomats back to Cuba, Rex Tillerson says, even as a new FBI report casts doubt on the theory that Americans there have been hit by "sonic attacks."

The unexplained illness, which lingered for months, bewildered a half-dozen neurologists in the United States.

The diplomats may have been targeted with a "sonic attack" at their homes, officials said.