General Ray Odierno, the Army chief of staff, said the Centers for Disease Control and Prevention is investigating what went wrong at Dugway Proving Ground, the Army site in Utah where the anthrax originated.
Government labs in Maryland and Virginia received the suspect anthrax, officials said. The rest were commercial labs in New Jersey, New York, Tennessee, Delaware, Texas, Wisconsin and California.
"At this time we do not suspect any risk to the general public," the CDC said in a statement Thursday.
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. US officials at Osan Air Base in South Korea said 22 people were being treated for possible exposure.
The anthrax samples were sent last month by commercial shipping companies to labs working to develop a new diagnostic test for anthrax, the CDC said.
There have been at least two other alarming incidents at Dugway, which has been testing chemical and biological warfare weapons since it was opened in 1942.
In 2011, the facility was locked down for 12 hours because less than one-fourth of a teaspoon of VX nerve agent was unaccounted for.
The results of a military internal investigation were never released. Utah Governor Gary Herbert said in 2011 that he met with the base commander and that the issue had been resolved to his satisfaction.
Dugway came under scrutiny in 1968 when 6,000 sheep died nearby. An Army report said the nerve agent was found in snow and grass samples. But years later, an Army spokesman said the cause of death of the sheep was never actually found.
