Julio V. Ruiz, a 71-year-old retired psychiatrist with a long history of participating in talks with the Cuban government, tried to ignore the persistent knocking at his door by two strangers when they showed up uninvited one afternoon last week.

The rapping on the door went on for 15 minutes. It was the F.B.I.

“Everyone tells you not to speak to them and to call your lawyer,” Dr. Ruiz said. “But you get scared. I was measured in what I said, and gave them a brief history of Cuba going back to the 19th century.”

At least five Cuban-Americans in Miami, including Dr. Ruiz, who have opposed a trade embargo with Cuba and promoted better relations with the communist government in Havana, said they received surprise visits in the past week from federal agents.

The law enforcement representatives were vague about their intentions, gave only their first names, and asked questions that seemed intended to learn about contacts with Cuban diplomats, Dr. Ruiz said.