Perhaps the first bioterror attack in this country occurred in 1984, perpetrated by the Rajneeshees - a cult from India. Its purpose was to gain political control of a county in Oregon. Afterwards, our government went to great lengths to cover it up. I don't know why.
My personal theory is that I believe that our government and the elites that control it are actually terrified by what they see as the rabble (meaning guys like you and me).
I think that they think that to acknowledge that we have attacked with biological weapons on our soil from outsiders would trigger a violent wave of vigilante justice against innocent dark-skinned people here at home.
I personally think that it's absurd, but to me it explains why these days the authorities always instantly rush forth to say "not terrorism" for pretty much every incident, even before they could possibly know for certain whether it was actually terrorism or not.
The whole story came out at the time. That's how you know about it. The government did not cover it up.
IIRC the purpose of the salmonella poisoning was to make enough citizens of the town sick so that they could not vote in the election. Therefore it was not really a "bioterror" attack.