Complex sentence: Forty Klansmen and Nazis, and several Communist marchers were involved in the shootings; sixteen Klansmen and Nazis were arrested and the six best cases were brought to trial first.
Split into simpler sentences: Forty Klansmen and neo-Nazis and several Communist marchers, were involved in the shootings whith sixteen Klansmen and neo-Nazis being arrested. The six best cases were brought to trial first.