A U.S. agency said it "may never know" how a hazardous type of avian influenza was sent to another site alongside a more benign strain, Reuters reports.
The more virulent virus might have been used by mistake or placed with the safer form via an unclean tool, but every virus culture involved in the March lapse "likely has been discarded," according to Monday comments by Tom Skinner, a spokesman for the U.S. Centers for Disease Control and Prevention. "We may never know exactly how cross-contamination occurred."
The center recently revealed the accidental transfer in a probe of a separate lapse, which led to scores of people at its Atlanta headquarters being placed on watch for potential anthrax infection.
The avian flu virus did not leap to any humans as a result of the incident, but CDC Director Thomas Frieden said the agent's threat to public health made its release "the most distressing" of several safety breaches at the center.
The health agency's influenza scientists found out about the accidental shipment in May. The compromised flu cultures went to the Agriculture Department, where chickens under study died unexpectedly.
Six additional weeks passed before top CDC officials learned about the breach, resulting in an Agriculture Department audit that wrapped up last Friday. The cause of the weeks-long reporting delay remains under examination.
The agency previously responded to the anthrax and influenza incidents by halting operations at two involved facilities. It also paused transfers of biological materials out of sensitive CDC sites prior to the completion of a re-assessment of protective protocols.