The U.S. Army's top general said Thursday that human error probably was not a factor in the Army's mistaken shipment of live anthrax samples from a chemical weapons testing site that was opened more than 70 years ago in a desolate stretch of desert in Utah.

Samples from the anthrax lot ended up at 18 labs in nine states and an Army lab in South Korea, leading more than two dozen people to get treatment for possible exposure.

Gen. Ray Odierno, the Army chief of staff, told reporters the problem may have been a failure in the technical process of killing, or inactivating, anthrax samples.

Odierno said the Centers for Disease Control and Prevention is investigating what went wrong at Dugway Proving Ground, the Army installation in Utah where the anthrax originated.

Officials said the government labs that received the suspect anthrax were at the Army's Edgewood Chemical Biological Center in Maryland and the Naval Surface Warfare Center in Virginia. The rest were commercial labs, which the Pentagon has declined to identify, citing legal constraints.

Some of the samples sent from Utah were also transferred to other labs in the U.S. by the Edgewood center, a research and development resource for nonmedical chemical and biological defense.

CDC spokesman Jason McDonald said four people at labs in Delaware, Texas and Wisconsin were recommended to get antibiotics as a precaution, although they are not sick. U.S. officials at Osan Air Base in South Korea said 22 people were being treated for possible exposure.

Odierno said normal procedures had been followed, and that he was not aware that such a problem had surfaced previously at Dugway.

But there have been at least two other questionable incidents at the military post 85 miles west of Salt Lake City that has been testing chemical and biological warfare weapons since it was opened in 1942.

In 2011, Dugway was locked down for 12 hours because less than one-fourth of a teaspoon of VX nerve agent was unaccounted for.

Military officials launched an internal investigation, but the results were not released. Questions about the incident were not answered Thursday by military officials. Utah Gov. Gary Herbert said in 2011 that he met with the base commander and that the issue had been resolved to his satisfaction.

In 1968, Dugway came under scrutiny when 6,000 sheep died nearby. An Army report acknowledged that the nerve agent was found in snow and grass samples, The Salt Lake Tribune reported based on a report that was declassified in 1978. An Army spokesman said in the late 1990s that state agriculture scientists never identified the cause of death of the sheep.

Herbert said in a statement he's concerned about the incident and is working with the CDC to monitor the investigation.

Test facilities like Dugway are intended to develop ways to defend against biochemical warfare, which some fear could be used by terrorists, said Barry M. Blechman, co-founder of the Stimson Center, a nonpartisan global security group in Washington.

He doesn't know why it was sending anthrax and is perplexed at how the mistake was made.

"This is an accident that should never happen," Blechman said. "You should have double-triple-positive controls over any live, lethal agent."

Steve Erickson, of the volunteer military watchdog group Citizens Education Project, said the incident isn't cause for panic but suggests more oversight is needed.

"Ever since 9/11, there's been a propensity to throw money at biodefense," Erickson said. "When you allow these activities to blossom and burgeon over a period of years without any effective oversight, you are asking for trouble."

The Dugway Proving Ground was created by President Franklin D. Roosevelt. The site has gone through name changes and been closed and reopened several times.

It sits on 180,000 acres of flat, desert terrain. It's miles from any population base, but it has a village with an elementary and high school, medical clinic, a few restaurants, a theater, pool, gym, and homes and temporary lodging.

About 1,700 people work at Dugway, including a mix of military and nonmilitary scientists, chemists, microbiologists and engineers.

It is one of six Army equipment-testing facilities that also include sites in Maryland, Arizona, Alabama and New Mexico.

In one of the chemical testing buildings at Dugway, chambers are used to test chemical warfare agents, the Army says in online materials. An outdoor range tests smokes, obscurants and explosives.

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Associated Press writer Brady McCombs reported from Salt Lake City.