A capable detective was assigned to investigate the cause of the tragedy.
Literal
Capable-[attributive-な] detective [subject-が] that tragedy-of cause [object-を] investigate-[attributive-する] duty [to-に] be-assigned-[passive-past-られた].
任務に当てられる ('to be assigned to a duty') is the formal way of describing job assignment. 当てる ('to apply, allocate, assign') is transitive; the passive 当てられる marks the recipient of the assignment. 悲劇 ('tragedy') and 捜査 / 調査 ('investigate') are formal vocabulary common in crime reporting.