Even if it was done by a disinfranchised government crank....
Still, it HAD to be embarrassing that a simple government worker with access to weapons grade anthrax could cause so much havoc.
Conclusion:
A) through poor record keeping and complete bureaucratic bumbling, they couldn't find the person responsible.
B) they knew exactly who did it, it was a paid psy-ops operation
C) during the hysteria they decided to keep the whodunnit under wraps so the very agencies "protecting us" wouldn't be responsible for not doing their tax-payer paid duty and look like fools.