AR 600-8-4 is the regulation for line of duty investigations.
Here is an extract that may assist you. I have also attached a copy of the regulation.
2–3. Requirements for line of duty investigations
Line of duty investigations are conducted essentially to arrive at a determination of whether misconduct or negligence was involved in the disease, injury, or death and, if so, to what degree. Depending on the circumstances of the case, an LD investigation may or may not be required to make this determination.
a. The LD determination is presumed to be “LD YES” without an investigation—
(1) In the case of disease, except as described in paragraphs c (1) and (8) below.