LE would find their blood on the machinery, shredders etc. So not the scrapyard IMO.I haven't looked at a single photo of either of these scrap yards so I'm making huge assumptions but my current mental image is that they were trying to hit one where catalytic converters specifically are stored and/or processed where precious metals and/or cash for/from catalytic converters and previous metals might have been stored. I'm assuming he shot them, presumably with an automatic rifle, then he transported their bodies to another site where there was heavy machinery and that the "dismembering" might have just been putting that entire vehicle bodies and all through a car crusher/shredder and then dump trucking the shreds into the river.
I'm probably wrong by some objective facts that can already be established but that's just where my imagination goes. It seems like the kind of process a scrap yard owner might think was foolproof and then feel legitimately betrayed by TV and movies when some of the (buoyant) body parts immediately floated to the surface and spread downstream to be found by searchers right away.