No one can make anything unless they have access to the correct spores. According to the article, a thousand spore samples from around the world were tested and only samples from the Armriid lab matched those used in the attacks. Of the 300 people who had access, relatively few were skilled enough in microbiology to prepare a highly refined spore prep. That should narrow the list of suspects.
According to the FBI, samples from USAMRIID and one other lab (presumably Battelle) matched the attack anthrax. There were eight samples with the same mutations as in the RMR-1029 flask and in the letters. Those eight samples were found at USAMRIID and at the other lab.