WASHINGTON—Pentagon officials said Wednesday the inadvertent shipment of live anthrax to laboratories nationwide stretched back a decade and that the scope of the mishap was likely to expand in coming days.

Four batches of the pathogen sent from the U.S. military laboratory at Dugway Proving Ground in Utah—which were thought to have been irradiated and inactivated—have been identified as containing live anthrax, officials said. Samples from those batches were sent to 51 sites in the U.S. and overseas over the past 10 years.

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