When it comes to shipping restrictions, the rules can be pretty complex: shipping alcohol is often frowned upon, shipping a car with more than 100 lbs. of extra cargo is not allowed, and transporting live animals in the mail is generally illegal.

Apparently, the U.S. government just ignored this last rule, however. The Pentagon admitted on Wednesday, May 27 that it had accidentally shipped live anthrax spores out of an army facility in Utah. The spores were inadvertently sent to 18 laboratories in nine different states, as well as a lab in South Korea.

According to reports from TIME and Huffington Post, Col. Steve Warren, the acting press secretary for the Pentagon, stated that U.S. officials had thought that the anthrax samples were dead.

The Los Angeles Times reports that at least 26 people are being treated for possible exposure to the live samples, although no infections have been reported yet. Four people have been given antibiotics as a precautionary measure, and the 22 others are being vaccinated and kept under surveillance to ensure that any symptoms are detected as soon as possible.

Anthrax is a bacterial disease and if left untreated, it can lead to death. A person can become infected by the bacteria if he or she inhales or ingests the bacteria, or if there is direct contact with skin. Most Americans became familiar with anthrax after terrorist groups sent dried spores in paper envelopes to politicians and media figures following the Sept. 11, 2001 attacks. The anthrax spores — which were dried, but not dead — ended up causing 22 deaths.

Dead samples are often used in research labs, and the U.S. Centers for Disease Control and Prevention stated that researchers had been using samples at the U.S. Army’s Dugway Proving Ground in Utah to test the bacteria against other chemical agents.

Government officials state there is an investigation into how the spores were sent out before being killed, and the CDC has also sent agents to each of the labs to collect and destroy the spores.