A Pentagon report about how anthrax was sent accidentally sent to labs in the U.S. and foreign countries says no root cause is identified. The report blames:
…insufficient technical information in the broader scientific community about how to make sure the potentially deadly substance was no longer live before it was disseminated to 86 labs in 20 states, the District of Columbia and seven foreign countries over the last several years.
The report does say that potential causes might include possible contamination of the samples, how much radiation was used to render the samples inactive, and the quality of the testing procedure. The Pentagon says there are no suspected or confirmed cases of anthrax infections as a result of the mistakenly shipped samples.