At least five workers developing an anthrax (search) vaccine at a children's hospital research lab in Oakland were accidentally exposed to the deadly bacterium because of a shipping mistake, officials reported Thursday.

Officials with the Children's Hospital Oakland Research Institute (search) said none of the researchers has shown symptoms of infection since the first exposure about two weeks ago, but each is being treated with precautionary antibiotics.

The researchers believed they were working with syringes full of a dead version of anthrax, hospital spokeswoman Bev Mikalonis said. Instead, they were shipped live anthrax by a lab of the Southern Research Institute (search) in the Frederick, Md., Mikalonis said.

Anthrax produces severe flu-like symptoms in most of its victims. If inhaled, ingested or otherwise introduced into the body, it can kill.

Other workers may also have been exposed while the researchers handled the live anthrax, Mikalonis said, a possibility that federal, state and local officials — including the FBI — are investigating.

Though the five workers were exposed, state health officials and the hospital don't believe anyone was infected because researchers took proper safety precautions.

The exposure doesn't pose a threat to patients because the Oakland lab is located about one mile from the hospital, according to officials.

"We do not see a threat or a danger to anyone in the community," said Dr. Richard Jackson, the California public health officer. "This really has been very well controlled."

The researchers are working with dead bacteria to develop an anthrax vaccine for children. Mail-borne anthrax attacks killed five people and sickened 17 others in 2001. Those attacks spurred research into better vaccines and treatments.

Mikalonis said the Oakland researchers received and stored the shipment from the Southern Research Institute, also known as SRI, about three months ago.

The researchers first used the tainted batch May 28 on lab mice which died soon after, hospital officials said at a news conference Thursday afternoon. But hospital officials said the head of the lab wasn't notified.

Then, last week, the researchers injected the anthrax into more mice. On Monday, those mice were found dead.

The researchers started their own investigation, and on Wednesday night, California state health officials confirmed that live anthrax was in the syringes. Agents with the FBI's bioterrorism unit removed the samples from the lab Wednesday, according to a hospital news release.

Southern Research Institute's Thomas Voss, who is in charge of homeland security and emerging infectious disease, said the Birmingham, Ala.-based nonprofit company is investigating. Voss said it's still unclear whether the institute did ship live anthrax.

"We aren't totally sure of the sequence of events," Voss said.

The Southern Research Institute has two highly secure "hot labs" that store some of the world's deadliest diseases. Labs and researchers from around the country that need data about those nasty diseases but don't — or can't — handle them contract SRI to do that work.

Voss said the institute's labs in Frederick and Birmingham handle just about every "select agent" listed with the Centers for Disease Control and Prevention. The institute is one of 350 entities registered with the CDC to handle live anthrax. It employs 600 people nationwide and has about $75 million in revenue a year, Voss said.

The mishap will likely be seized on by critics of the government's effort to combat biological terrorism by paying for the construction or expansion of 18 high-containment labs nationwide. Supporters of the building boom said the additional lab space is needed to combat emerging global threats, but critics said such expansion increases the likelihood of accidents.

"This is exactly the kind of thing that a lot of groups that oppose this spate of construction fear," said Edward Hammond of the Sunshine Project, a chemical and biological weapons watchdog group. "This is the type of accident that has concerned them a lot."