"The case against Ivins just gets weaker."

This strikes me as important:

Anthrax investigation still yielding findings: Chemical composition of spores doesn't match suspect flask/Nature

Furthermore, the Bacillus subtilis contaminant present in the letters has not been matched to the strain found in [Bruce] Ivins' lab. The FBI speculates it might have come from another lab at Fort Detrick, but has not yet identified any lab that has it. The case against Ivins just gets weaker.