They were acquitted after a change of venue requested by the defendants. That is what led to the riots.
Later there was a federal case brought against them for deprivation of civil rights, and that is where they were convicted, but there was no change of venue in the federal case.
Thanks, I only remembered the second half. The Feds stepped in to make two trials for the same thing legal. Double jeopardy double smesherdy, pshaw.