Mohammad Saaili Shibin, also known as Khalif Ahmed Shibin, 50, was also found guilty in federal court in Norfolk, Va., of negotiating on behalf of the pirates who seized the Marida Marguerite, a German-owned vessel, in 2010, prosecutors said.
U.S. Attorney Neil MacBride described Shibin as the highest-ranking Somali pirate convicted so far.
The Quest, a U.S. yacht with two couples on board, was hijacked in 2011. Jean and Scott Adam of Marina del Rey, Calif., the boat's owners, and their friends, Bob Riggle and Phyllis Macay of Seattle, were killed when U.S. military forces tried to retake the vessel.
Shibin was convicted of a long list of charges, including conspiracy, hostage-taking and violence against a vessel at sea. He is to be sentenced Aug. 13.