Complex sentence: The FBI, which did not usually handle missing-persons cases, became involved in the investigation regarding Dele along with the French authorities and concluded that Dele, Karlan and Saldo were probably killed, and then thrown overboard, by Dabord.
Split into simpler sentences: The FBI, which did not usually handle missing-persons cases, became involved in the investigation regarding Dele. However, the FBI and French authorities became involved in the case and concluded that Dele, Karlan and Saldo were probably killed, and then thrown overboard, by Dabord.