"There is an open investigation," U.S. Customs special agent Robert Fernandez told the newspaper. "If there's a U.S. citizen, U.S. resident or U.S. entity involved, it would be a violation."

The Herald said the Cuban government is a major supplier of dolphins for water parks, where tourists typically pay up to $150 to swim with and touch the popular marine creatures.

The United States imposed an economic embargo against communist-run Cuba four decades ago. Havana has struggled for new sources of foreign exchange after an economic crisis precipitated by the collapse of the Soviet Union, its major financial sponsor, a decade ago.

The capture of wild dolphins for use in water parks has long been a target of American animal rights activists. U.S. law permits their capture but pressure by rights groups brought about a voluntary moratorium a decade ago.

But Cuba's Ministry of Science and Technology and its National Aquarium in Havana have moved into a lucrative market for dolphins in the Caribbean and Europe, the Herald said.

The chief marine mammal veterinarian at the National Aquarium is revolutionary Che Guevara's 38-year-old daughter, Celia, the paper said.

According to figures submitted to a United Nations agency that compiles information on the animal trade, Havana has sold 82 dolphins in the past five years, making Cuba the world's leading exporter. Newly captured Atlantic bottle-nosed dolphins bring between $40,000 and $70,000 on the international market, the paper said.

Animal rights activists told the Herald that the U.S. investigation has focused on Graham Simpson, who has dual U.S. and British citizenship, and his wife Pam Pike, an American, who started dolphin attractions in Antigua and Anguilla.

Simpson said he contracted for dolphins with a broker in the Dominican Republic and was not told that Cuba was the supplier until just before delivery.

"I thought of myself as a British citizen living for the last three years in Anguilla, which has no law against buying from Cuba. It really didn't occur to me this might be a problem," he told the Herald.

Violations of the U.S. embargo against Cuba can bring corporate fines up to $1 million and tens of thousands of dollars in individual fines.