U.S. defense officials say the problem of unintended shipments of potentially live anthrax spores over the past decade is likely worse than first believed.

The Centers for Disease Control and Prevention said Wednesday it is investigating what the Pentagon called an inadvertent shipment of live anthrax spores to government and commercial laboratories in as many as nine states.

At least 52 workers at the Centers for Disease Control and Prevention are taking antibiotics as a precaution because of a lab safety problem that may have accidentally exposed them to anthrax.