On June 9, two weeks since this story first burst forth, we are up to 68 labs that likely got some live anthrax from Dugway. And another country has been added, the UK.
If the story is accurate as told so far, what probably happened is that there was incomplete inactivation due to inadequate doses of gamma irradiation. Those responsible for the inactivation process (usually exposing spores to cesium or cobalt sources of gamma rays) may not have recalibrated the radiation dose for larger collections of spores, and for different containers, media, etc. Bacterial killing is not like killing cows. A cow is either alive or dead. But collections of bacteria die at log rates, and killing that very last spore can require a much higher dose of radiation than is needed for the first 99.99% of the batch.
The good news is that it takes, in general, many thousands of inhaled spores to produce an infection. So small numbers of live spores won't generally make anyone sick. Though even one can be used to grow a deadly batch of new anthrax spores.