If this was a bulk mailing there would be registration info gathered by the PO. Even if the letters where put in the mail stream without going through bulk mail the post office puts ID tags on the backs of letters that ID the machine, time of day, etc. for the place where the letters where processed. If a business produced the letters, the letter head may lead to the printer. If produced in a home on laser or inkjet there is a chance that there will be micro-printing from the printer that would provide more info. If the FBI has already determined Seattle area and they really are pursuing this there may be enough evidence to find these folks.
I have such little faith in Holder and Obama that it is depressing.