WASHINGTON — The FBI on Monday offered reporters a detailed look at the science behind the investigation of the 2001 anthrax mailings, which resulted in five deaths. Genetic signatures of the bacteria were prominent clues that eventually led the investigators to two Erlenmeyer flasks at the U.S. Army Medical Research Institute of Infectious Diseases at Fort Detrick in Maryland.

The investigation had implicated Army microbiologist Bruce Ivins as the perpetrator of the anthrax mailings. Ivins committed suicide last month while under investigation. An affidavit released by the FBI earlier this month described Ivins as “sole custodian” of the batch of spores having the telltale DNA.

But at Monday’s briefings officials conceded that about 100 people had access to the same anthrax batch, called RMR-1029. One other institution, which the FBI would not name, also had anthrax with the same genetic signature, investigators said.

The FBI had obtained an anthrax sample having DNA that linked Ivins to the mailings in early 2002, but the sample was later destroyed, said chemist Vahid Majidi, assistant director for the FBI's Weapons of Mass Destruction Directorate, who led the briefing with Christian Hassell, director of the FBI Laboratory.

Four mutations lurked in the DNA of the anthrax culturing in the flasks, the same mutations that were found in samples of the anthrax mailed in the 2001 attacks, the panel reported. While mutations naturally arise in bacteria, especially over the course of several generations, the four mutations the scientists homed in on were stable, said Claire Fraser-Liggett, one of six researchers who discussed the science side of the investigation during two press briefings Monday.

“They are not random mutations that come and go,” said Fraser-Liggett, director of the newly created Institute for Genome Sciences at the University of Maryland School of Medicine in Baltimore.

There was no evidence that anything was added to the spores of the rod-shaped bacteria to make them disperse more easily, Majidi said. Preliminary tests suggested that some of the mailed spores contained silica and oxygen, resulting in speculation that the spores were mixed with something that would make them extra buoyant and perhaps more dangerous. But transmission electron microscopy localized the silica signal to inside the spore coat, said Joseph Michael, a materials scientist at Sandia National Laboratories in Albuquerque, N.M. The bacterium may naturally incorporate environmental silica into its spore coat as it develops, the researchers said.

Spores of Bacillus anthracis easily drift through the air and take on charge, which makes them stick to everything, said James Burans, associate laboratory director at the National Bioforensic Analysis Center in Frederick, Md. That’s why labs typically work with anthrax only in liquid form. “People describe it as having a mind of its own,” Burans said.

All of the anthrax mailed in the attacks was identified as belonging to one strain, the Ames strain, which is used in several labs doing basic or vaccine-related research, said Paul Keim, a microbiologist at Northern Arizona University and director of the Pathogen Genomics Division at the Translational Genomics Research Institute. But a closer look at cultures grown from spores recovered from the mailings revealed phenotypic variation — differences in color, size and texture — that hinted at underlying genetic variation that could help researchers distinguish between batches of spores.

Led by Fraser-Liggett and Jacques Ravel, also of the Institute for Genome Sciences, researchers fully sequenced 12 samples of anthrax from the mailings, with the hope that DNA would lead back to the mother stock. Four mutations — specific insertions or deletions known as “indels” that appear in the genetic code of many organisms — were identified as significant. To determine which labs were using stock with these same four mutations, investigators obtained more than 1,000 samples of Ames strain anthrax from 16 labs in the United States and some in Canada and Sweden.

Ivins consulted with investigators in 2002 regarding the sampling protocol that should be outlined in the subpoena for collecting anthrax samples, the panel reported. But Ivins then submitted a sample without following the requested lab protocol. The FBI destroyed this sample, not because it was tainted, but because all samples needed to be collected in exactly the same way in order to hold up in court. New samples submitted by Ivins did not contain the four mutations.

Later, investigators realized that Keim, whose lab was keeping a backup of every sample collected, might have the backup of the original sample Ivins submitted. This sample did have the four mutations, investigators reported Monday. Other samples from Ivins’ lab confirmed this finding. The panel would not speculate why Ivins would have submitted two different samples.

Researchers were mum about many of the specifics, saying the results eventually will appear in peer-reviewed journals.

The investigation was seminal in establishing the field of microbial forensics, said microbiologist Rita Colwell, who was director of the National Science Foundation at the time of the attacks.

The panel refused to comment on the more gumshoe detective aspects of the case but said it was unprecedented that the FBI and Department of Justice hold a briefing on a case that has yet to go to trial. Scientists involved in the case wanted to set some of the scientific record straight, Majidi said.

“I don’t think we will ever put all suspicions to bed,” said Majidi. “There’s always going to be a spore on the grassy knoll.”

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