Unfortunately for the attackers, they screwed up ~ the letters they so carefully mailed in Florida ended up in mail transport equipment (MTE) thought to be empty.
Rather than spreading anthrax spores through airport baggage and express operations, several airplanes, postal facilities and important government buildings, the letters got carried by ground transport and bypassed almost everything but 3 main postal facilities and a US Senate mailroom.
This attack was not a "warning" ~ it was a full scale attack against the main postal operations in the capital city of the United States of America, the Congress, and the main baggage and express operations in New York City, and Washington DC.
It's roughly the equivalent of a severing of the head!
The question is, why was that anthrax attack a one-time bad deal? We responded to 9/11 by overthrowing the Taliban in Afghanistan and (IMHO) we responded to the anthrax attacks by invading Iraq. Why was it possible to attack us once but not again?