Whoa. Good thinking. Check the envelope flap for saliva and DNA (unless a sponge was used to dampen the glue or the envelopes were self-sealing). They could also look for some Middle-Easterner, who was murdered (before developing anthrax.) or was given Cipro. Come to think of it, I wonder if Kathy Nguyen was asked by anyone to lick envelopes.
I am sorry if this is too complex for the FBI, but they are the ones who keep saying that if there is no letter involved her infection cannot be explained.
You will notice I just treated her infection as a trivial problem - readily solved by anyone who knows anything about letter and flat trays and how the post office really works. Frankly I am positively amazed that not a single solitary FBI agent ever worked his way through college as a PTF - NOT ONE!