To reiterate with the sash, which seems to a piece of bedding from the apartments, I believe we may be able to glean some information off of the positioning of the blood spatter, if it was indeed used to be a sort of shield for the attacker to avoid getting blood on themselves as they attacked Dia. For instance, the height of the attacker was called into question with this positioning.

I do have the fabric with me, as I know some wanted to see it with their own eyes.

[Even though he is more than confident with what he's talking about!! But whatever.

He produces the makeshift sash, blood, dust and all. Feel free to have it be passed over, etc.]

I believe we should consider the involvement of dust a bit further as well, namely why Dia's fingers appear to be dusty, and the potential of dust transfer while handling that fabric.

[He's even had to brush himself off a few times. He's fairly certain whoever was handling it wouldn't get away from it without getting dusty themselves.]