I don't believe they ever found the source of the Florida anthrax. There were lots of false leads but I don't think the actual source was found. Just a little contamination in the mailroom and on the computer keyboard of the guy who died.
Maybe someone who remembers the facts better can correct that.
In any event, the October 9 letter attack was quite different. It was a much more serious attack using more refined anthrax. It may have been a copycat crime to the Florida attack and from an entirely different person.
The different grades of anthrax used in the various attacks have been a curious and confusing development.