Authorities are investigating the disappearance from a New Jersey bioterror research lab of at least three mice carrying a deadly strain of plague.
Sources say FBI agents and bioterrorism experts have interviewed and polygraphed employees at the University of Medicine and Dentistry in Newark, N.J., the location of the lab run by the Public Health Research Institute, a leading center for research on infectious diseases.
The mice have been missing for approximately two weeks.
"The FBI responded to the matter, and we dedicated a great number of agents as well as a large number of resources to the investigation," said Special Agent Steve Siegel, a spokesman for the FBI's Newark field office.
"We're satisfied that there is no public safety risk, and there doesn't seem to be any nexus to criminal activity or terrorism," he added.
Nevertheless, federal authorities, including the FBI, have criticized the lab for lax procedures that resulted in a potential public health menace.
"This is the black death," said Richard Ebright, a microbiologist at Rutgers University. "This is the disease that killed a quarter of Europe's population."
Officials discovered two weeks ago a failure to account for three of 24 mice that had been injected with a bacterium that causes various forms of the plague, including bubonic plague, inside the high-security facility located in the middle of the city of Newark.
The injections were part of a government funded bio-defense project to develop vaccines against biological weapons of mass destruction.
The Public Health Research Institute concedes the missing mice are a problem.
"Even though we process 10,000 animals and we can account for nearly all of them, the fact that we couldn't account for three are three too many," said Dr. David Perlin, a researcher at the institute.
The discovery that three of the mice were missing led to a full investigation by the FBI Joint Terror Task Force, and an ongoing investigation into the lab's safeguards by the Centers for Disease Control and Prevention, federal sources said.
The investigation included polygraph tests and extensive interviews with all employees of the lab who had access to the three cages where the mice were kept.
Investigators concluded that lab employees failed to properly account for the mice. They believe that when the mice in two of the three cages died, lab workers failed to properly search the cages' bedding before it was incinerated and the mice were packed for biohazard disposal.
The other mice, which survived the exposure to the plague bacterium, also were destroyed after the completion of this phase of the test. They and their bedding also were not properly safeguarded, the investigators found.
Investigators still do not know what happened to the three missing mice.
There is a very slim chance that the mice were able to escape the facility, investigators said. If they did, sources involved in the case estimated their time of survival and potential transmission of the disease at three days.
Surveys of hospitals in Newark, where the lab is located, have turned up no evidence of human infection.
"We don't have any indication there's a public health threat," said Von Roebuck, a spokesman for the CDC. "We're basing that on the fact that we've had no reports of the plague in the area."
If human cases of plague do turn up -- as they sometimes do naturally in the wild -- countermeasures will be taken, Roebuck added. If diagnosed early, for instance, bubonic plague can be treated with antibiotics.
But regardless of the health threat in this case, Stephen Albert Johnson, director for the Center for Innovations in Medicine at Arizona State University's Biodesign Institute, said the missing mice incident raises questions about increased federal bioterror prevention research.
"Risk [from the missing mice] is low, but this is a very bad incident in regard of control," Johnson said. "One of the implications of our bringing a lot more people and money in to work on threat agents is that more of these types of incidents are likely to happen."
Researchers in federal bioterror facilities already are given background checks and have to go through an approval process for safety and security protocols, Roebuck said. In addition, the CDC conducts site visits to ensure labs are secure and following protocol.
In addition, Roebuck added, the CDC continues to investigate the missing mice to determine what, if anything, can be done to avoid a repeat of the incident.
ABC News' Pierre Thomas, Richard Esposito, Roger Sergel and Michael S. James contributed to this report.