Furthermore, at a critical juncture in the story we're informed that it was, in fact, the elderly couple's own son who loaded the murder weapon. However, since it's still too soon at that point to reveal that the son and Ronald Opus, the shooting victim, are one and the same, it's likewise too soon to reference the suicide note. Since the information has to be attributed to someone, we are told, most implausibly, that "a witness" had seen the son loading the gun six weeks earlier. But, realistically, who other than his own parents would have been in their apartment that fateful day to observe Ronald Opus loading shells into the shotgun?
Such inconsistencies give us ample reason to distrust the story as it was told to us, so let's go to Dr. Don Harper Mills, past president of the American Association for Forensic Sciences, for the final word:

