The 2001 anthrax letter attacks panicked the nation. Ten years later, researchers are finally able to discuss how they identified the origins of the deadly bacteria, which they were able to trace to an Army research lab in Maryland.
The anthrax attacks in September 2001 killed five people and sickened 17, the worst biological attack in U.S. history. Anthrax spores (a dormant form of the bacteria) were sent in letters to Sens. Tom Daschle and Patrick Leahy and several news organizations.
The intensive investigation, dubbed "Amerithrax" by the FBI, fingered Bruce Ivins as the main suspect. Ivins was a researcher at the government's biodefense labs at Fort Detrick, Md., where he had access to anthrax, but some question the evidence against him. Ivins took his own life as the FBI was preparing its case.
Anthrax is a naturally occurring bacterium found in the soil that can infect the lungs, the skin or the intestinal tract. If inhaled, anthrax is nearly always fatal – up to 90 percent of patients die unless the infection is caught early.
Identifying the source
The researchers found that the anthrax enclosed in the envelopes was related to theB. anthracis Ames ancestor, a lab strain not commonly found in nature. The FBI collected Ames samples from the letters as well as from different labs and environments around the world and handed the samples to University of Maryland researchers to identify which ones came from the same source.
"The FBI was very careful to keep everything secret from us that we didn't need to know," University of Maryland researcher Steven Salzberg said. "Our job was to sequence those genomes and tell them if we found differences and which samples they were."
Salzberg and his colleagues first cultured the samples and noticed that while they appeared the same in spore form, four of the samples had slightly different-looking colonies when matured. They sequenced these oddballs and noticed that each sample had the same set of four genetic mutations.
"There were four particular isolates [colonies] with their own unique genetic markers that were all found together in one tube at Fort Detrick," Salzberg told LiveScience. "These four different types were in this test tube, and all four were also found in three of the letters."
There are over 5 million nucleotides (which come in four types) in the genomes of anthrax, and so it is very unlikely that different cultures developed the same three mutated strains independently. "The likelihood that happened by chance is vanishingly small," Salzberg said. "It can really only happen if the samples came from the same source."
The researchers handed over that information to the FBI and didn't hear back for five years, until the FBI announced that the source was most likely an anthrax sample at Fort Detrick labeled RMR-1029, one that many researchers had access to.
Salzberg stands by his genetic work, though his team didn't have any say on the link established between Ivins and that vial. In fact, that link has been called into question by a report released by the National Academy of Sciences in 2010. The National Academy group said the evidence supporting the theory that Ivins sent the letters was circumstantial and wouldn't stand up in court. Many researchers had access to the lab strain, and there was a lack of physical evidence tying Ivins to the letters, the group said.
"A goal of microbial forensics, in an overall investigation of who is responsible and who is not, is to provide a piece of the puzzle," Steve Schutzer, of the University of Medicine and Dentistry of New Jersey and editor of the second edition of the textbook "Microbial Forensics" which was released in 2010, told LiveScience in an e-mail. "The paper represents scientists' efforts to provide a solid piece of the puzzle."
The paper was published March 7 in the Journal Proceedings of the National Academy of Sciences.
You can follow LiveScience staff writer Jennifer Welsh on Twitter @microbelover.