WASHINGTON—Investigators believe live anthrax samples may have been sent inadvertently from an Army research facility to as many as 18 labs in nine states and South Korea, officials said Thursday.

The U.S. military has ordered 22 service members and Defense Department civilians in South Korea who may have come near the live samples to take the antibiotic Cipro as a precautionary measure.

"There are no suspected or confirmed cases of anthrax infection in any of these personnel," said Col. Steve Warren, the chief Pentagon spokesman.

In addition to South Korea's Osan Air Base, the defense lab at the Dugway Proving Ground in Utah also sent samples of anthrax to the Naval Surface Warfare Center in Dahlgren, Va., and the Edgewood Chemical Biological Center in Maryland.

The Pentagon hasn't identified private labs that received the shipment, nor said whether their employees are being treated as a precaution. But U.S. officials said there are no confirmed case of anthrax infection in anyone who came in contact with the samples. The civilian labs are in Texas, Wisconsin, Delaware, Tennessee, California, New York and New Jersey.

The military ships both live and inactive anthrax samples by FedEx, sealed inside absorbent material and surrounded by dry ice, said a military official. The samples are also placed inside two containers to prevent any accident leakage.

While the packaging is the same for active and inactive material, live samples are supposed to be tracked by the Centers for Disease Control and Prevention and are subject to enhanced security by FedEx, the official said.

FedEx didn't respond to specific questions but said in a statement it is working with the Pentagon to gather information about the anthrax shipments. "FedEx is committed to the safe transport of all customer shipments, and our priority is the safety of our employees," company spokeswoman Connie Avery said in a statement.

A civilian lab in Maryland learned Friday that a sample it had obtained from the Dugway defense lab more than a year earlier contained live samples, not inactive spores. The U.S. government was notified that night, and the Pentagon let the other 18 labs that had received similar samples Saturday, said Col. Warren.

Col. Warren said the Pentagon quickly assessed there was no danger to the public, then began looking into what happened.

"We got the information out as rapidly as we could," Col. Warren said.

At a breakfast briefing with reporters sponsored by the Center for Media and Security, Gen. Ray Odierno, the Army chief of staff, said the live anthrax spores survived the process that was supposed to kill the pathogen.

"We followed all the procedures," Gen. Odierno said. "The best I can tell, it was not human error."

Write to Julian E. Barnes at julian.barnes@wsj.com

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