Oh I think in a high profile case like this, a court order could be expedited. Every piece of mail nowadays is scanned by a scanner and an image is saved of the envelope showing the address and postage. It wouldn't be that difficult to seach through OCR'd images to find a letter sent from the killer or sent to a media outlet from that particular post office.
Well, perhaps. But, I guess what it actually does show —is that in these situations things don’t work in the same way as one would think they should. The confusion, I guess, tends to muddle things...