Bacterium infected 3 at BU biolab

Page 3 of 3 -- "The deck was stacked against [the researchers] because they were working with something they had no idea they were working with," Moore said.

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But Moore acknowledged that researchers in the lab had violated policies requiring them to work with tularemia inside an enclosed box, called a hood, that sends air through sophisticated filters.

Instead, the tularemia samples were sometimes worked with in the open, in part because the enclosed research boxes were sometimes filled with material that should not have been kept there, Moore said.

Blood tests were performed on about 60 university researchers, and those tests showed that only the three workers who had become ill tested positive for tularemia. After the exposure was determined, BU in November shuttered the lab for decontamination. The part of the lab where the tularemia research was conducted remains closed.

Eleven researchers were placed on paid leave in November, to ensure the integrity of the investigation, and six remain off the job.

The investigation into how the exposure happened continues. Samples of tularemia were sent directly from the Nebraska lab for CDC analysis, and those tests showed no presence of the dangerous strain, deepening the mystery around the episode.

"At this time it seems to me there's no evidence conclusively to link the contamination to Boston or to Nebraska," said Jennifer Morcone, a CDC spokeswoman. "Certainly, everyone would like to determine the source of the contamination to make ceratin nothing like this could happen again."

Infectious disease specialists at other universities said that BU appeared to have acted properly by notifying health agencies. They also said that BU was not obligated to alert the public to the exposure of researchers.

"I don't know what the point would be of telling the public because there was no danger to the public," said Karl Klose, a professor of microbiology at the University of Texas at San Antonio.

As a result of the exposures, BU as well as the Boston Public Health Commission are moving to tighten oversight of research. To improve safety in the dozens of public and private research labs in Boston, the Public Health Commission intends by this spring to start a mandatory training program for lab workers, emphasizing the reporting of illnesses in researchers. The commission also plans to hire a lab safety inspector who will make unannounced visits to research sites to make certain they are following safety protocols, said John Auerbach, Public Health Commission executive director.

Alice Dembner and Beth Daley of the Globe staff contributed to this report. Stephen Smith can be reached at stsmith@globe.com.