It looks like the source for the claim that the seat residue was PETN is Sanders' interpretation of the testing he had done on the stolen seat fabric. That testing was done by
West Coast Analytical Service, which is in Santa Fe Springs, CA (Jetblast has said the lab was in NM, probably mistakenly reading it as Santa Fe). Does anyone have information about the results of those tests? All I could find was this from a
very pro-missile web site. Apparently the lab concluded the residue was not consistent with explosives, though Sanders continued to think it was. Here is the quote:
So no lab who tested the residue said it was consistent with explosives, the guy who stole the fabric does not think it was explosive residue, but a reporter thinks it is anyway. And that is literally the entire case in favor of the residue being related to explosives, as far as I can see. Correct me if there is more to it.