Spores from the Sterne strain of anthrax bacteria (Bacillus anthracis) are pictured in this handout scanning electron micrograph (SEM) obtained by Reuters May 28, 2015. REUTERS/Center for Disease Control/Handout via Reuters

The number of labs that received live anthrax samples from a U.S. military base in Utah has risen to 66 in 19 states, Washington and three foreign countries, the Pentagon said on Monday.

Army Colonel Steve Warren, a Defense Department spokesman, said 31 people were receiving precautionary treatment for exposure to live anthrax, unchanged since the Pentagon's last update on the incident.

Warren said one new state, Pennsylvania, had been added to the list of locations where live anthrax had been sent. In addition to the U.S. states and Washington, the pathogen was sent to South Korea, Canada and Australia.

Investigators are trying to ascertain whether the inadvertent shipments of the anthrax, which can be used as a biological weapon, stemmed from quality control problems at the base, the Dugway Proving Ground, Pentagon officials said.

No one at the labs has been infected and there is no known risk to the public, officials said.

A number of U.S. military facilities for the past decade have shipped what were supposed to be inactivated, or killed, anthrax samples to outside labs to develop countermeasures to biological weapons. The one in Utah is the only one known to have sent samples that proved to have live bacterial spores.