Was Pentagon's shipment of live anthrax "human error"?

The anthrax scare involving the U.S. military is more serious than first thought. More than two dozen people may have been exposed to live anthrax, which can be deadly.

Over the past two months, anthrax was sent via FedEx from a U.S. Army laboratory in Utah to 18 government and private laboratories in nine states and to a U.S. base in South Korea.

U.S. military admits to shipping live anthrax

The anthrax spores, which were being studied in an effort to find better ways to defend against anthrax, were supposed to be dead. But last Friday, a lab in Maryland notified the Pentagon and the Centers for Disease Control (CDC) that the anthrax spores it received were, in fact, still alive.

Inhaling live anthrax can be lethal.

The CDC is in the process of testing the other 18 batches sent, but presumes they are alive too.

Florida Professor Kenneth Berns CBS News

The CDC says there is no risk to the public. But at least four people who were exposed to the anthrax in the U.S. and 22 people who were exposed to it in South Korea are being provided antibiotics.

The CDC says that the antibiotics are only a precaution and so far, no one has shown any anthrax symptoms.

On Thursday, General Ray Odierno, the Army Chief of Staff, said the Utah base followed normal procedures. He added, "The best I can tell, it was not human error."

University of Florida Professor Kenneth Berns, however, says human error is the most likely explanation. He says they should have double checked their work.

"They should have been tested to see if, in fact, they really were inactivated. And my guess is that probably wasn't done adequately," Berns tells CBS News.

The CDC says the risk to the public - including FedEx workers who handled the unopened packages - is almost zero.

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    Chip Reid

    Chip Reid is CBS News' national correspondent.