The chief of the US government's top public health agency acknowledged a pattern of safety errors Wednesday after dangerous mixups in the handling of influenza and anthrax.

Last week, the Centers for Disease Control and Prevention acknowledged five incidents over the past decade -- two of them in recent months -- in which workers shipped anthrax, flu, botulism and a bacteria known as brucella to other labs without following proper de-activation and safety procedures.

"I think we missed a critical pattern," CDC chief Tom Frieden said during two hours of questioning from the House Energy and Commerce Subcommittee on Oversight and Investigations.

"The pattern is an insufficient culture of safety."

No one was believed to have been hurt by the mishaps, but they exposed a major lapse of protocol within the CDC, which is viewed globally as a leading scientific and health agency.

The discoveries included the mistaken contamination of a mild flu strain with a dangerous H5N1 bird flu that was shipped to a US Department of Agriculture poultry lab. The incident happened six weeks before it was made known to CDC leadership.

Other problems included the potential exposure of around 80 workers at the CDC's Atlanta headquarters to anthrax in early June, when samples were not properly handled and deactivated before shipment.

The CDC issued a report Friday that detailed three other lab mistakes: two in 2006 involving live anthrax and botulism, and one in 2009 involving brucella, a strain of bacteria that can cause the infectious disease brucellosis.

More details emerged shortly after Frieden's testimony regarding the discovery earlier this month of six forgotten vials of smallpox at a separate US government lab at the National Institutes of Health, which had also raised alarm over the potential release of dangerous biological agents that could be used as terror weapons.

The Food and Drug Administration, which was responsible for the lab where the smallpox was found, issued a statement saying a total of 327 vials of biological samples, including dengue, influenza, rickettsia and Q fever, had been found in the same cold storage area.

"This collection was most likely assembled between 1946 and 1964, when standards for work with and storage of biological specimens were very different from those used today," the FDA said.

"Overlooking such a sample collection is clearly unacceptable."

- Anthrax in food storage bags -

A separate investigation by the USDA has revealed more problems at the CDC, according to a memo about the report released by lawmakers earlier this week.

The probe found there were missing containers of anthrax that had to be tracked down by inspectors. Some materials were also transported using only food storage bags, and anthrax was stored in unlocked refrigerators in a hallway where workers passed through freely.

"What in heaven's name would go through the minds of some scientists, thinking a Ziploc bag is enough to protect someone from anthrax?" asked Representative Tim Murphy, a Pennsylvania Republican who chairs the House subcommittee that hosted the hearing.

Frieden said anyone who handled anthrax that way would have done so because he or she believed it had been inactivated, and he promised to be directly involved in the investigation and the implementation of safety changes.

"While we have scientists who are the best in the world at what they do, they have not always applied that same rigor that they do to their scientific experiments, to improving safety," said Frieden.

The CDC has shut down two labs and issued a moratorium on the shipment of dangerous agents from its facilities until a thorough review can be completed.

Frieden said he has appointed a single point person to oversee safety and was working to convene an internal review board as well as an external advisory group to offer ways to prevent such dangerous incidents in the future.

Congresswoman Kathy Castor, a Democrat from Florida, said there have been at least 14 separate reports, letters and lab investigations from various US government branches documenting safety lapses and lack of oversight at CDC high containment labs over the last decade.

"It appears that CDC has not heeded those reports," Castor said.

"It's troubling. I mean, this has gone on for years now."