They say no.
The affidavit says:
Bacillus subtilis is a nonpathogenic bacterium found ubiquitously in the environment.
In other words, it's everywhere. And there are too many variations for anyone to keep track of.
In the roundtable discussion they said, "it really didn't drive us any place specific."
That tells me that, either they found it all over the place, or they realized it couldn't be used to prove anything, so they stopped looking after awhile.
It could also be like the hairs found in the mailbox. The odds that the hair would be from Ivins' head were probably a million to one against. But they had to test. When it was confirmed that it wasn't Ivins' hair, it proved nothing. But the MEDIA had headlines: "Hair Samples in Anthrax Case Don't Match."
Sometimes it's better not to go down a path that you know will only lead to confusion and misunderstandings -- particularly when you know there are much better paths to travel.
There are only 24 hours in a day. It's not humanly possible to explore every possible option to everything.
I should have added: That's why we have so many conspiracy theorists. The conspiracy theorists can always argue that IF the FBI had gone all the way down path #1,276,859,204 it is possible it would have led directly to the person they believe was responsible for the anthrax attacks.