The Pentagon on Friday added Japan to a list of countries that received live anthrax samples from the US military, while a news report said the lab that sent out the bacteria faced sanctions in 2007 for failing to kill specimens but was never fined.
The sample that went to Japan was sent to the US military base of Camp Zama about 25 miles (40 km) southwest of Tokyo in 2005 and was destroyed in 2009, Defence Department spokesman Colonel Steve Warren said.
Warren said the anthrax, which was sent to Japan for the purpose of testing detection equipment, came from a master lot that was thought to have been inactive but turned out to be active when tested.
"It's important to note that there currently is no anthrax, activated or inactivated, in Japan at this time," Warren told a regular Pentagon news briefing.
Five countries outside the United States are now known to have received live anthrax samples: Australia, Canada, South Korea and Britain, as well as Japan. Sixty-nine labs in 19 states and Washington DC, also received live samples.
Investigators have been trying to determine whether the inadvertent shipments of the live anthrax, which can be used as a biological weapon, stemmed from quality control problems at a US base in Utah, the Dugway Proving Ground, Pentagon officials said.
In an online report on Friday, newspaper USA Today said that in 2007 lab inspectors from the Centres for Disease Control and Prevention cited Dugway for using an experimental chemical method of killing anthrax specimens that was ineffective and resulted in an unauthorised transfer of the bacteria to another facility.
It quoted the CDC as saying that inspectors found that Dugway staff apparently ignored the results of their own kill-confirmation tests that showed growth of bacteria even after the chemical inactivation was done - and shipped the sample anyway.
The CDC referred Dugway to the US Department of Health, but the department's Office of Inspector General did not issue any fines even though it agreed that an unauthorised transfer of anthrax had occurred, USA Today said.
It said the CDC had noted that the 2007 case involved a chemical method of inactivating anthrax, not the gamma irradiation method that is the focus of the current probe.
Following the revelation that anthrax was shipped abroad, North Korea has accused the United States of targeting it with anthrax. In a letter made public on Friday, it asked the United Nations to investigate Washington's "biological warfare schemes."