Owned and run Americans who were engaged in the slave trade:

Seizing the opportunity, American merchants based in Rio de Janeiro sold U.S.-made vessels to slavers sailing for Africa. The proliferation of the American ship and flag — used by slavers of all nationalities — in the illicit commerce soon provoked alarm among diplomats.

“Our flag is preferred over any other,” complained Gorham Parks, the U.S. consul in Rio de Janeiro, in an 1848 letter.