Complex sentence: One of those witnesses was young Gene Crowder, who mysteriously disappeared under unknown circumstances, while Kate's foreman, Frank Buchanan, had a shoot-out with unknown suspects before disappearing as well; both were presumably murdered by Canton.
Split into simpler sentences: One of those witnesses was young Gene Crowder, who mysteriously disappeared under unknown circumstances. Another, Kate's foreman Frank Buchanan, had a shoot-out with unknown suspects before disappearing as well; both Buchanan and Crowder were presumably murdered by Canton.