Complex sentence: While the majority of these cases are believed by local authorities to be the result of accidental drownings, Gannon and Duarte have theorized that the young men were all murdered, either by an individual or by an organized group of killers.
Split into simpler sentences: While the majority of these cases are believed by local authorities to be the result of accidental drownings, Gannon and Duarte have theorized that the young men were all murdered. Either by an individual or by an organized group of killers.