i wonder if they found any evidence of sexual assault on Shirley, or if she was just too decomposed to say conclusively. If she was nude when her body was recovered, that would imply that she was raped. And that's information that LE is not going to be willing to reveal to the public until later on.
As sick as it sounds, I think the killer(s) may have taken Shirley with the intent of sexually assaulting her. Not necessarily that the whole crime was committed to assault her, but the abduction part. I can't think of another reason why they'd kill the man and take the woman, unless the killer had a personal beef with Shirley.