US personnel working at an Army facility in Utah appeared to follow correctly all the outlined procedures to inactivate anthrax before they mistakenly shipped off live samples of the deadly bacteria, the Army's top general said on Thursday.
Army Chief of Staff General Raymond Odierno said investigators were now reviewing the procedures themselves to determine why the bacteria was not rendered inactive.
"The best I can tell there was not human error," Odierno told reporters, cautioning that his information was based solely on preliminary reports.
US officials disclosed on Wednesday that US Army facilities mistakenly shipped live anthrax bacteria to laboratories in nine states and an air base in South Korea.
The Pentagon has said there was no known suspected infection or risk to the public.
But four US civilians have started taking preventive measures called post-exposure prophylaxis, which usually includes the anthrax vaccine, antibiotics or both.
Twenty-two personnel at the base in South Korea were also given precautionary medical measures although none have shown sign of exposure.
The US Centers for Disease Control and Prevention (CDC) has begun an investigation of the incident. Odierno said the CDC was reviewing whether "we have to change the procedures that are in place."
Odierno did not offer specifics and said he did not know how long the procedures had been in place.
"But we definitely believe no one is in danger. We believe we followed all the proper procedures," Odierno said.
The mishap comes 11 months after the CDC, one of the government's top civilian labs, similarly mishandled anthrax. Researchers at a lab designed to handle extremely dangerous pathogens sent what they believed were killed samples of anthrax to another CDC lab, one with fewer safeguards and therefore not authorized to work with live anthrax.
Scores of CDC employees could have been exposed to the live anthrax, but none became ill.
That incident and a similar one last spring, in which CDC scientists shipped what they thought was a benign form of bird flu but which was actually a highly virulent strain, led US lawmakers to fault a "dangerous pattern" of safety lapses at government labs.
In the latest case, the Army's Dugway Proving Ground in Utah reported in March 2014 that gamma irradiation had inactivated the anthrax stock in question, and along with another Army facility, began shipments that continued through April 2015, a US official said.