The Anthrax Case: The Trail of the Spores

WASHINGTON, D.C.--Microbiology and genetics helped lead the Federal Bureau of Investigation (FBI) to U.S. Army researcher Bruce Ivins, but they didn't clinch the case of the anthrax letter attacks of September 2001, according to an FBI press briefing held here this morning. The briefing detailed the genomic analysis that was used to trace where the anthrax came from, and it also dispelled the myth that the spores had been combined with silicon to make them into a deadlier weapon. The final and exclusive link to Ivins, however, depended on low-tech detective work about which the agency is still keeping mum.

The briefing was led by Vahid Majidi, a chemist who heads FBI's Weapons of Mass Destruction Directorate, and D. Christian Hassell, director of the FBI Laboratory, along with six outside scientists who helped with the investigation. The officials said preliminary analysis of the mailed anthrax at the U.S. Centers for Disease Control and Prevention showed that it wasn't a uniform population of cells but included a handful of phenotypes of the same strain that varied slightly in texture, color, and size. That launched investigators on a hunt for a stock of anthrax where the spores had come from. In order to do that, researchers led by Paul Keim at Northern Arizona University in Flagstaff extracted DNA from the various phenotypes and handed it over to scientists at The Institute for Genomic Research (TIGR) in Rockville, Maryland, where 12 DNA samples were fully sequenced.

Of the many mutations discovered in the samples, FBI scientists decided to home in on four distinct mutations that were the most likely to be passed on from one generation of cells to the next (ScienceNOW, 12 August). "These were not random mutations that come and go" but are mutations that "remain stable over a long time frame," says Claire Fraser-Liggett, the former head of TIGR, who was present at today's press conference.

FBI scientists then worked with outside collaborators to develop assays to look for the four mutations in a repository of more than 1000 samples of the Ames strain collected from labs in the United States, Sweden, the United Kingdom, and Canada. Only eight samples had all four mutations, and all were traced to RMR-1029--the flask of spores under Ivins's charge--and to a few labs outside of the U.S. Army Medical Research Institute of Infectious Diseases. FBI investigations eventually ruled out everyone but Ivins, Majid said. He refused to give details of the investigative process that had led to this validation but said "the simple check of a lab notebook could be one way to do it; shipment records would be another way to do it."

Other scientific work done by materials researcher Joseph Michael at Sandia National Laboratories in Albuquerque, New Mexico, convinced the FBI that silicon had not been added to the anthrax in the letters. Although preliminary analysis done at the Armed Forces Institute of Pathology had indicated the presence of silicon, transmission electron microscopy by Michael and his colleagues revealed that the silicon was contained inside the spores--a natural occurrence documented in previous research--rather than a coating intended to make the anthrax more easily dispersible.