Federal officials are still at a loss to explain how a potentially deadly strain of influenza could be sent to more than 4,000 labs around the world.

The Centers for Disease Control and Prevention is operating under the presumption that the H2N2 strain was purposefully included in the panels designed to test the labs' proficiency in identifying viruses.

"I'm sure it was not an inadvertent use," said Dr. Julie Gerberding, CDC director, "because it would be almost impossible to believe that they didn't know they were dealing with H2N2."

The samples were sent, beginning in September, as part of a testing process that measures a laboratory's proficiency in detecting various strains of influenza. The College of American Pathologists directs the testing and contracted with Meridian Bioscience Inc., a company based in Cincinnati, to distribute the test panels.

The organization has a policy of excluding micro-organisms that can harm people from the test panels.

Dr. Jared Schwartz, an officer with the organization, said Meridian thought it had sent an ordinary flu strain, meaning a strain for which there are vaccines readily available. He said Meridian found the virus in 2000 in a "germ library" that had come from another company.

Meridian executives were traveling and not available to comment, spokeswoman Brenda Hughes said.

Gerberding said she believes the strain was included in the test kit because it grows well and can be easily manipulated in the lab yet "without really considering that even a test strain in a panel could potentially cause a hazard, not only to the workers in the lab but to the people in the community."

CDC officials said the likelihood of the virus getting out in the public is remote, and that there are no signs anyone has contracted it.

"If an unusual virus had emerged, we would have known it by now," Gerberding said.

Still, she said, the agency was intent on ensuring that every sample shipped to more than 4,000 labs in 18 countries or territories had been destroyed.

The World Health Organization's influenza chief, Klaus Stohr, said he was "relatively confident" most of the samples outside the United States would be destroyed by Friday.

Canada, South Korea, Hong Kong and Singapore have destroyed their samples, while Japan was doing the same, WHO said. Taiwan and Germany announced they had destroyed all their vials.

The germ, the 1957 H2N2 "Asian flu" strain, killed between 1 million and 4 million people. It has not been included in flu vaccines since 1968; anyone born after that date has little or no immunity to it.

Gerberding said the CDC would work with the pathologists and other organizations to establish better guidelines for proficiency testing.

Congressional action is also possible.

Senate Majority Leader Bill Frist, R-Tenn., a heart-lung surgeon, said the shipments underscored "the need to bolster America's domestic and global public health infrastructure."

Rep. Edward Markey, D-Mass., said the virus shipments raises concerns about how they could be used by terrorists.

"We need a strong system to be put in place to limit the access to these materials and reduce the potential for an accidental or an intentional release of pathogens that pose a serious health hazard," Markey said in a letter to Michael Chertoff, the secretary for homeland security.

Mike Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, said he agreed that Congress should review how the strain was so easily distributed.

"We can't have this happen," Osterholm said. "Who needs terrorists or Mother Nature, when through our own stupidity, we do things like this?"