With all due respect, I believe it is plausible that the killer was on a planned mission and may or may not have known or cared. He seems to have been determined and prepared to meet and prevail over any and all challenges and certainly had an unobtrusive path in and out. IMOOr, it speaks that the killer knows them well enough to know they’re not armed. I’m stating the obvious there I know.
That said, he may have surveilled them for a limited time before the attack from the back wooded area.
And I can’t imagine he would be carrying a cell.