Scientists reveal how culprit in 2001 anthrax attacks was found

Key forensic evidence in the 2001 U.S. anthrax attacks has been revealed. The FBI had previously prevented the scientists involved from speaking publicly about their findings in case this interfered with court proceedings, but last August, after chief suspect Bruce Ivins committed suicide, the case collapsed and the FBI lifted many of the restrictions. Debora MacKenzie writes that this week, some of the scientists involved revealed their results at a scientific meeting in Baltimore, Maryland.

These show how the FBI traced the spores used in the attacks to a single flask at a U.S. government lab, but they do not explain why the FBI made Ivins — who worked at the U.S. Army Medical Research Institute for Infectious Diseases (USAMRIID) - the chief suspect. In late 2001 envelopes containing dry anthrax spores were sent to a number of US media outlets and politicians, leading to five deaths. Later that year, Paul Keim at the Northern University of Arizona in Flagstaff identified the anthrax bacterium used in the attack as the U.S. army’s Ames strain. The FBI then obtained 1,072 anthrax samples from the 18 labs it knew to have Ames and got several research groups, including Keim’s, to compare their genomes with that of the strain used in the attacks. The hope was this would uncover mutations that would finger one lab as the source.

McKenzie writes that Keim and his colleagues told the Baltimore meeting that initial reports that useful mutations had been found were misleading. The full genome sequences revealed “no genetic differences at all,” says Keim. Instead, the researchers say, the key clues came from a lucky discovery. A technician, also at USAMRIID, had noticed patches of unusual-looking spores in cultures of the attack anthrax, and recultured just those. Keim and colleagues sequenced their genomes and found ten mutations that differed from the common Ames sequence. Because the spores made up a fraction of the total, these “minority” mutations hadn’t shown up initially.

Next the team developed highly sensitive tests to screen all 1,072 samples for four of the mutations. Eight samples had all four. One came from a flask labeled RMR-1029 that Ivins was responsible for at USAMRIID. The other seven came from cultures taken from that flask, only one of which was not located at USAMRIID. So while these findings show the attack spores came from one of these cultures, the FBI has gone further in concluding the attack came directly from the RMR-1029 flask.

There was another question: how did the attacker turn the water-based slurry of spores in the flask to the fine, dry powder in the letters?

Joseph Michael of the Sandia National Lab in Albuquerque, New Mexico, used specialized electron microscopy to show that 75 per cent of the attack spores had incorporated silicon into their coats while growing. As spores taken directly from RMR-1029 following the attacks had no silicon in their coats, and the other seven genetic matches had either none or a lower percentage, the attack spores must have been recultured before they were posted.

During this process, they would have shed their coats, multiplied, then turned back into spores. Was Ivins’s level of expertise needed to turn these recultured spores into dry powder? “What I am hearing is that the spores in the letter were not special. It would not take a lot of time or equipment to make them,” says Keim. Michael’s images show the attack anthrax contained spore clumps, unlike professionally produced powders.

The FBI may have evidence to show Ivins was the link between RMR-1029 and the envelopes, though with civil suits from Ivins’s and the victims’s families pending, the bureau would not be revealing it soon. For now, the researchers say their studies nail the spores as coming from the flask, but not the identity of the attacker.