Let's look at the paint on the bra (and the hammer) to see if we can't work out what it means.
I speculate:
The large amount of paint on the bra cup indicates she had no shirt on. Unless paint pots were transported to the desert but they're photographed in the house and not in the graves.
She was wearing the bra when it got paint on it because she's still wearing it in the grave.
I reject the suggestion she was painting while lying on the floor and paint dripped down onto her - this wasn't a drip or splash from above, it's a thick coating of her left cup (see photo below) and then she's turned over onto her right side and the paint has dripped down towards the middle of the bra, suggesting it happened soon after the paint got there because it hadn't dried, and there was no more movement after that because she's lying there long enough for the paint drips to dry and not to change course.
Her left bra cup was submerged in the paint tray - to get that dripping quantity and I think that could be a line of paint showing submersion. I can't think of another source of paint in an area large enough for the side of her body to lie submerged in, like the paint tin for example.
This attack happened in the house while she is wearing bra and sweatpants. The paint on the bra is matched to the same paint on the hammer, and the paint was wet when it got on the hammer. Unless there was a wet paint source ie paint tin, roller, or tray, in the desert the hammer got paint on it at the house and the wielder of the hammer had wet paint on his hands or gloves.
There is no blood in the white paint on the hammer.
This suggests the hammer wasn't washed - if it was soluble paint. The type of paint in the tray was probably being used with a roller suggesting it was paint for the walls, not for the woodwork, and so it was likely to be an emulsion (washable) paint.
So where is the blood on the handle if it hasn't been washed? Does this indicate it was not a very bloody scene -
no blood spatter on the paint on the hammer or on the killer's gloves which made contact with the paint, and it's a very short handle close to site of impact. Does the fact the handle wasn't washed suggest he was using someone else's hammer and wearing gloves, so he wasn't worried about fingerprints in the paint or DNA?
If I try and tie this in with my earlier posted theory about Summer's wet pants and being restrained with painters tape
while seated, I would suggest the head strikes match that scenario perfectly. I theorize she was struck first under the chin to get her to give him her phone password, breaking her jaw, and then later when he decides to kill her he hits her multiple times on the head while still restrained seated upright, because if she was on the ground and unconscious after the first head strike he would not have access to all sides of her head unless he's turning her over to hit her but that seems pointless.
Multiple strikes to the head seated in the same position, lead to her death and the release of urine. Remember Susan Blake talking about the smell of dirty diapers - maybe he forgot to clean up the seat of the chair cushion. He goes and finds a large container in the closet - so that there is no DNA in the truck.
When the restraining tape is removed she falls to her left side on the floor and the paint tray. She is rolled over as he removes her from the paint tray and sees her pants are wet and maybe fecal-soiled and he doesn't want it on his clothing because of the odor so he pulls her pants off by the left leg forcing the right pants leg to pull up inside. He dumps the pants in the container as well as the ball of painters tape and then her body lying on her right side and the paint starts to drip down on her bra. The right cup is still attached to the bra because there's no point cutting one half of her bra off only to leave the other side still on her. She's lying in the grave with her left side down so tipped out of the container from her right side exactly as she went into it, pants ending up last next to her head. I think the uppermost right bra cup in the grave has been ripped by pulling from hard earth by animal activity.
Photos - click to enlarge.
1. the bra cup
2. the paint on the hammer handle
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