Federal authorities said they are investigating an apparent laboratory foul-up in which live anthrax bacteria were shipped from Maryland to California by Federal Express because scientists involved in the transfer thought the bacteria were dead.

The potentially lethal germs were sent by Southern Research Institute, of Frederick County, to a private lab in Oakland about three months ago, officials said. The mistake came to light recently when dozens of laboratory mice died after vaccine researchers in Oakland injected them with anthrax bacteria that supposedly had been chemically deactivated.

Tests by the California Department of Health Services confirmed Wednesday that the germs were alive.

Federal medical officials are now examining what went wrong when the Southern Research Institute shipped 22 cubic centimeters -- or about four teaspoons -- of anthrax bacteria to the Children's Hospital Oakland Research Institute.

The bacteria were intended for research underway at the Oakland laboratory to develop a children's vaccine for anthrax contamination. The bacteria were kept in storage until late last month. There were no signs of tampering in what was the third shipment from the Maryland laboratory to the Oakland facility in the past year. The two previous shipments of anthrax bacteria had been chemically deactivated.

On May 28, researchers injected 10 mice with the supposedly inert bacteria. All were dead by June 3. Then, researchers injected 40 mice on June 7. They died three days later, and tests proved that the bacteria had been the cause.

"Basically, at this point, we have something that's unexplained," said Michael G. Murray, director of infectious disease at Southern Research Institute. "In fact, it was shown to be dead."

Although tests came back negative yesterday for seven lab workers in Oakland who were possibly exposed to the bacteria, all have started on a 60-day regimen of the antibiotic Cipro as a precautionary measure, lab spokeswoman Beverly Mikalonis said.

Bioterrorism experts in the FBI's San Francisco office assisted in removing the toxins from the California lab, FBI spokesman Bill Carter said. He said there is no indication of criminal activity.

The federal Centers for Disease Control and Prevention is trying to determine what happened, Carter said.

In an era of heightened security against terrorism, the federal government has tightened regulations on researchers using biological agents and toxins. The CDC is charged with overseeing registration of all such facilities, including government agencies, universities and commercial researchers.

"All I know is that we're working with all the institutes involved to find out what happened and make sure it doesn't happen again," CDC spokeswoman Karen Hunter said.