A drug investigation initiated by the son of Liberia’s president resulted in two convictions and two acquittals Thursday in a case that a prosecutor said was brought against one of the oldest drug distribution networks in the world.

The jury in U.S. District Court in Manhattan convicted Chigbo Peter Umeh, described as the mastermind of a plot to corrupt top Liberian officials, and Konstantin Yaroshenko, a Russian pilot. It acquitted two Ghanaian men who the government said were enlisted to transport drugs by boat.