(Reuters) - Scientists at the U.S Centers for Disease Control and Prevention are conducting tests to see whether the procedure they followed to kill anthrax, though flawed by their own standards, may nevertheless have killed the potentially deadly pathogen before it was sent to less-secure laboratories, where employees work without adequate protective gear.

If they are right, it may mean dozens of scientists and staff, who were given a vaccine and powerful antibiotics to prevent anthrax infection, may never actually have been in danger of getting the potentially deadly illness that was at the center of 2001 bioterror attacks.

"We are still figuring out exactly what happened," CDC Director Dr. Thomas Frieden said in an interview with Reuters. "It's not clear at this point whether infectious material actually left the high-containment lab."

Researchers in the CDC's bioterrorism response lab are retracing the events there between June 6 and June 13 that led to the possible exposure of 84 employees at the agency's Atlanta campus, Dr. Paul Meechan, director of the CDC's environmental health and safety compliance office, told Reuters.

New details about the agency's investigation suggest the anthrax that was being inactivated in a high security lab may have been sitting in a bath of acid for 24 hours before being transferred to two lower-security labs.

Researchers are trying to find out whether that was long enough to kill the anthrax, Meechan said in a telephone interview.

"We don't know that, but we're doing experiments to prove it," said Meechan. The CDC first disclosed the incident to Reuters last week. (Full Story)

An independent laboratory is running the same set of experiments to see if they get the same answers, which would add to the validity of the findings.

Meechan said workers in the bioterror lab had been testing a new protocol for inactivating anthrax before sending the bacteria for experiments in two lower-security CDC labs.

CDC researchers had been using that protocol to inactivate other bacteria, but not anthrax. It called for placing anthrax into a bath of acid for 10 minutes, removing some, putting it on a nutrient-rich plate and placing it in an incubator.

After 24 hours, the researchers checked to see if any colonies of anthrax had grown. None had, so the team took the anthrax that had been soaking in acid for 24 hours, put it on slides and sent it for testing in two other CDC labs.

The material from the 10-minute sample eventually germinated and started to divide and form a colony, a process that normally takes around 48 hours.

SOAKING IN ACID

Why the team did not wait the standard 48 hours to be sure the acid bath had killed the bacteria is still under investigation, Meechan said.

Investigators want to learn what was happening to the anthrax cells left in the acid bath while the material from the 10-minute sample was in the incubator.

"We want to know whether or not in the 24 hours when they were waiting for that plate to grow, they were actually killing more of the anthrax, and possibly all of it," Meechan said.

A CDC team is setting up an experiment using a similar setup, taking samples of anthrax soaking in acid at intervals of up to 24 hours.

"The idea is to see how much time it takes to kill everything in that solution," Meechan said.

Results of the studies will be available soon, said CDC spokesman Tom Skinner.

The agency is also using a detailed questionnaire to assess each employee's proximity to the lower-security labs that may have been handling live anthrax to determine his or her personal risk of exposure. And the CDC has taken samples of surfaces within labs that received the material to determine whether live anthrax was present.

All of those efforts will help the CDC determine whether employees need to continue taking antibiotics to prevent anthrax, Skinner said.

Meanwhile, the CDC has tasked Dr. Harold Jaffe, its associate director for science, with conducting a separate investigation into the circumstances surrounding events that led to the mishandling of anthrax, which is on a list of pathogens considered a select biological agent because it could threaten public health and safety.

"The fact that it was even a concern or that it might have happened is unacceptable," Frieden said. "Employees should never have to be concerned about their safety from preventable exposures."

He has asked for Jaffe's report to be on his desk by early next month.

Meanwhile, he said, inspectors from the U.S. Department of Agriculture's Animal and Plant Health Inspection Service are at the CDC this week conducting their own investigation.

(Reporting by Julie Steenhuysen; Editing by Cynthia Osterman and Lisa Von Ahn)