A chain of blunders in the processing and handling of lethal biological agents compelled Congress to call in the head of the Centers for Disease Control and Prevention to explain what went wrong and what's being done to ensure that the one of the world's leading health organizations never encounters such systemic failures again.

The congressional panel was said to have presented years worth of watchdog reports on dangerous miscues in the CDC's operations that involved an absence of care in handling live samples of smallpox, anthrax and bird flu. The hearing was before an oversight committee of the House Committee on Energy and Commerce.

Tom Frieden, director of the CDC, said the center's recent failures were being addressed by tighter oversight and a cultural change across the center's laboratories. He said the CDC was working to develop enhanced measures to prevent additional failures, outbreaks of disease and biological terrorism -- but he also accepted accountability on behalf of himself and the CDC for its past lapses.

"While we have scientists who are the best in the world at what they do, they have not always applied that same rigor to safety," Frieden said. "In hindsight, we realize we missed a crucial pattern: a pattern of incidents that reflect the need to improve the culture of safety at CDC."

In a pair of egregious lapses, two CDC labs were said to have transferred what they thought were inactive samples of anthrax to other sister centers. The anthrax was still active and the receiving laboratories were ill-equipped to handle the active samples.

In another incident, a CDC laboratory was said to have accidentally contaminated a weaker strain of bird flu with a stronger variant, before shipping the sample off to a Department of Agriculture facility. When used in testing, the virus wiped out all of the chickens.

In yet another incident, watchdogs discovered that an active, unguarded container of smallpox remained at a National Institutes of Health facility for decades. The pathogen was supposed to have been transferred to a CDC laboratory during the 1950s.

Some of the CDC's other miscues included using expired disinfectants to clean up its anthrax issues, failing to educate its employees on the proper containment procedures for weaponized agents, and recurring issues with with its air containment system at its headquarters.

Rep. Fred Upton, R-Mich., chairman of the full House Committee on Energy and Commerce, stated at the hearing that the CDC's sequence of failures evidenced a larger issue and asked why the panel should believe things have changed.

"A dangerous, very dangerous pattern is emerging and there are a lot of unknowns out there," Upton said. "Why do these events keep happening?"