ATLANTA – The Centers for Disease Control and Prevention (CDC) closed two laboratories after officials discovered a series of safety breaches.

Officials at the agency said they learned that researchers at CDC's influenza lab cross-contaminated a less-harmful strain of animal influenza with highly pathogenic H5N1 influenza. The incident occurred as researchers were preparing to ship the less-harmful strain to a partner laboratory.

"And for me personally, this is the most distressing of the three incidents for two reasons," said CDC Director Tom Frieden during a July 11 news conference. "First, because it happened in our influenza laboratory; and second, because it happened six weeks ago, and I learned about it less than 48 hours ago."

CDC shipped the samples to a US Department of Agriculture laboratory. Researchers at the USDA lab" saw that the strain was not behaving as a low-pathogenic strain would behave," Frieden said. "And they therefore did tests and identified on May 23 or thereabouts that it was contaminated with H5N1. They informed the CDC laboratory staff, the CDC laboratory staff did testing to confirm that, in fact, it was the specimen we sent there that was contaminated and that when we got the strain into our laboratory, it wasn't contaminated, so it happened here."

Frieden noted several steps he initiated to improve laboratory safety and best practices. For example, CDC implemented a moratorium on biological material leaving BSL-3 and BSL-4 laboratories at the agency. Also, Dr. Michael Bell was named director of laboratory safety. Dr. Bell will serve as the single point of accountability for all laboratory safety protocols, practices and procedures.

"And I’ve established a high-level working group chaired by Dr. Bell that will report to me to accelerate improvements in laboratory safety and serve as the transition group for permanent single point of accountability on laboratory safety which is something called for in the review of the potential exposure to anthrax incident," Frieden added. "The culture of laboratory safety needs to improve at some CDC laboratories."

CDC will also re-examine procedures used to inactivate bacteria and viruses before samples are transferred from CDC labs.

Additional steps taken include disciplinary action for staff members who knowingly failed to follow lab safety protocols or who knew about lab safety incidents and didn't immediately report them.

Two other laboratory mistakes led to CDC researchers being exposed to anthrax, and vials of smallpox were found on the National Institutes of Health campus in Maryland. Frieden said NIH staff quickly secured the specimens and transported them to Atlanta.

"There was no risk to workers or to the public, but this event also should never have happened," Frieden said.