This story says a "stray spore" was found on the keyboard.
http://www.usatoday.com/life/2001-10-15-tracking-anthrax.htm
Where is that scientific report showing all of the floors of the AMI building??? They showed the keyboard was loaded? It was a screening of the building.
http://www.ph.ucla.edu/EPI/bioter/anthraxamibuilding.html
I think it was more than a stray spore.