A top Pentagon official has revealed that at least 4 batches of anthrax samples sent to a number of laboratories contained live spores.

The shipments were sent from a military lab to 51 laboratories in 17 U.S. states, as well as 3 foreign countries. The Pentagon is still investigating how the potentially deadly samples were sent out, and will publish its results by the end of June, write Alison Young and Thomas Vanden Brook for USA Today.

Pentagon Says Live Anthrax Samples Sent To 51 Labs

Method of deactivating anthrax spores not 100% effective

Top officials have been charged with finding out how the leak occurred, and preventing a recurrence of the mistake. The military lab at the Dugway Proving Ground was supposed to have sent out dead specimens of anthrax.

“The process of inactivating them is kind of a delicate one,” said John Peterson, a microbiology professor who works with anthrax at the University of Texas Medical Branch at Galvestonhe. Some material similar to the spores must be left behind for testing.

The spores produced by Bacillus anthracis, the bacterium that causes anthrax, can cause serious illness if they are inhaled. The Dugway lab apparently used radiation to kill the spores, although chemicals can also be used.

To check whether the specimens have been killed, a small amount of the pathogen should be left in an incubator for a week in order to see if any spores are left alive.

Dugway errors part of a wider problem

Officials would not provide details about the deactivation method used at Dugway. The mistake was discovered by an unidentified private biotechnology company in Maryland. Their report sparked a huge international investigation, with many questioning how Dugway and other labs did not notice that the spores were live.

Investigators have since stated that Dugway’s deactivation method was not completely effective. “We have concern that the inactivation procedures, when followed properly, are inadequate to kill all spores, and the U.S. government is developing an approach to securing such possible samples from misuse,” wrote Daniel Sosin, deputy director of CDC’s Office of Public Health Preparedness and Response. USA Today obtained the email, which Sosin sent to state officials.

The mistakes at Dugway are just the latest in a string of accidents at laboratories since summer 2014. The Pentagon specified that the live spores were sent out by mistake, and not as part of a terror attack.