WASHINGTON (AP) — The United States issued an ominous warning to Americans on Friday to stay away from Cuba and ordered home more than half the U.S. diplomatic corps, acknowledging neither the Cubans nor America's FBI can figure out who or what is responsible for months of mysterious health ailments.

No longer tiptoeing around the issue, the Trump administration shifted to calling the episodes "attacks" rather than "incidents."

The U.S. actions are sure to rattle already delicate ties between the longtime adversaries who only recently began putting their hostility behind them. The U.S. Embassy...