(CBS News) — Six American tourists have mysteriously died in recent months at resorts in the Dominican Republic. The FBI is helping local authorities investigate these deaths.

Robert Wallace traveled the world with his wife and friends say he worked hard and played even harder. But his niece said he fell ill almost immediately after drinking a scotch from the minibar at the Hard Rock Hotel & Casino and was dead just three days later in April.

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David Harrison of Maryland died at the same resort last year. He and his wife had taken their 12-year-old son to the Dominican Republic to celebrate their wedding anniversary. The Hard Rock Hotel & Casino said it’s confident that all operational protocols were followed.

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Miranda Schaup-Werner and Yvette Monique Sport died at the Bahia Principe resort. Their death certificates list the cause of death for both as heart attacks. But Felecia Nieves, Sport’s sister, questions that.

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