Forty Cuban migrants sneaked ashore in Florida as a training exercise designed to deal with a possible mass exodus from Cuba was under way, officials say.
The operation had been planned before Fidel Castro fell ill
They were found on two beaches. A US border patrol spokesman said the case enhanced the realism of the exercise.
The two-day Operation Vigilant Sentry, which ended on Thursday, involved 325 agents and 85 law enforcement agencies.
Most Cubans who reach US shores are allowed to stay - those intercepted at sea are usually returned to Cuba.
Over two days, hundreds of law enforcement agents enacted a scenario which imagined thousands of Cubans fleeing violence in the event of the death of President Fidel Castro.
Officials said the training exercise had been planned before Mr Castro fell ill last July.
But the operation took on a renewed urgency when he temporarily handed over power to his brother, Raul.
President Castro recently spoke in a live broadcast with his Venezuelan counterpart Hugo Chavez, but has not been seen in public since last year.
Coast Guard Rear Adm David Kunkel said: "It's a mass migration plan in general. It doesn't have to be from Cuba.
"However we do recognise that Cuba is certainly an area where we must be prepared."