The Army mistakenly sent live anthrax samples from a testing facility in Utah to commercial laboratories in as many as nine states, including California, as part of an effort to improve field testing for biological threats.
Pentagon officials said the accidental transfer of the potentially deadly biological agent Bacillus anthracis, better known as anthrax, had not caused any known infections.
"There are no suspected or confirmed cases of anthrax infection in potentially exposed lab workers," Col. Steve Warren, a Pentagon spokesman, said in a statement.
"At this time we do not suspect any risk to the general public," the CDC said in a statement.
The CDC said it had sent investigators to all the labs and was trying to determine whether they also received live samples.
Officials said the commercial laboratories are in California, Delaware, Maryland, New Jersey, New York, Tennessee, Texas, Virginia and Wisconsin. They did not identify the specific labs.
The Pentagon said one sample of anthrax also was sent to Osan Air Base in South Korea. A program there aims to boost biosurveillance capabilities on the Korean peninsula.
Under military research programs, anthrax spores must be inactive before they are sent to labs for study.
In this case, live spores were accidentally sent from the Army's vast Dugway Proving Ground, about 85 miles southwest of Salt Lake City, to labs working to develop a new diagnostic test for anthrax.
Dugway is used to test defense systems for chemical and biological weapons agents, including lethal viruses and bacteria.
In 2011, Dugway was put on lockdown overnight when a vial of deadly VX nerve agent went missing. The vial later was found, but had been mislabeled.
The nation's worst biological attack involved anthrax created in an Army facility.
Weeks after the Sept. 11, 2001, attacks, five envelopes containing anthrax spores were sent to several members of Congress and the media, sparking widespread fear of another act of terrorism.
At least 22 people contracted anthrax, and five died from the infection. About 35 post offices and mail rooms were contaminated along with seven buildings on Capitol Hill.
After years of false starts, the FBI concluded in 2008 that Dr. Bruce Ivans, a researcher at the Army's Medical Research Institute of Infectious Diseases in Frederick, Md., was responsible. He committed suicide before he was charged.