The Pentagon wasn't able to find a single root cause for shipments of live anthrax to labs worldwide, but it did identify poor procedures for inactivating and testing the deadly virus.

The Defense Department issued a 38-page report Thursday detailing its investigation into the shipments earlier this year.

Live anthrax was sent from a Defense Department lab in Utah to 66 labs spread across 19 states, the District of Columbia and three foreign countries. The live spores don't pose a threat to the public and no one has died from the outbreak.

The report found that over the past decade, shipments of live anthrax have been made to four more foreign countries.

The report did not find a single root cause for shipping the viable anthrax specimens to labs for further research on countermeasures to the virus.

However, the committee investigating the shipments did find a slew of problems with the testing of samples, contamination prevention and radiation dosing used to kill the anthrax spores.

These deficiencies and other factors led to the "establishment of protocols that do not completely or permanently sterilize these samples," the report said.

Some labs lacked written procedures for reporting and finding contamination in the lab, according to the report.

Part of the problem is the Defense Department didn't take enough samples for testing and used a very short period from when the samples were irradiated and then tested for viability.

It is clear that anthrax spores "are particularly difficult to kill and live spores injured by irradiation may be able to repair their injuries over time," the report said.

Another problem was the procedures for biological safety aren't consistent across all laboratories. In most cases, the committee observed that agency labs just followed their own protocols and procedures, the report noted.

To combat this problem, the committee called for the Pentagon to improve quality-control programs at all labs and standardize the procedures for irradiation and viability.

The agency should do a better job of figuring out the exact dose of radiation needed to kill the anthrax spores. More testing also should be completed to ensure the spores are properly irradiated, the report said.

Since the incident, the Defense Department stopped shipping inactivated anthrax samples from several labs. The agency advised labs that may have received any inactivated anthrax that they should stop working on it.