The scientific evidence by itself can't pinpoint the origin of the anthrax mailings that in 2001 and infected 22 people and killed five, according to a report from the National Research Council.

While the evidence the Federal Bureau of Investigation found good leads, there was no way to conclude absolutely where the anthrax used in the mailings came from.

The anthrax was mailed to Sens. Tom Daschle and Patrick Leahy, both Democrats. Letters containing anthrax were also mailed to ABC News, NBC News, CBS News, The New York Post and the National Enquirer. The mailings started on Sept. 18, 2001.

In 2005 the FBI's probe focused on Bruce Edwards Ivins, a senior researcher in biological weapon defense at the U.S. Army Medical Research Institute of Infectious Diseases. Ivins committed suicide in 2008, a week before the FBI announced that he was the prime suspect in the case. Sen. Leahy at the time disputed the FBI's conclusions.

Anthrax kills its victims by generating toxins as it reproduces. There are three forms. One affects the skin, another the digestive tract and a third attacks the lungs. The first is usually not fatal, especially after treatment with antibiotics. The second kills about 40 percent of those infected depending on whether and how they are treated. The respiratory form has mortality rates approaching 90 percent without treatment.

The NRC report says that the FBI correctly identified the type of anthrax that was used, called the Ames strain of Bacillus anthracis. While the strain of anthrax was genetically similar to a variety found at the USAMRIID labs, it was not necessarily derived from that strain and other possibilities, such as parallel evolution, were not explored.

Further, the strain used in the letters would have had to undergo further processing if it was in fact the USAMRIID strain. In addition, the contents of letters sent to New York and to Washington were different.

One thing the report says would have been a great help are new molecular analysis techniques, some of which were developed afterwards.

The committee commends the FBI for reaching out to the scientific community for assistance early in the anthrax letters investigation, said Alice P. Gast, chair of the committee and president of Lehigh University, in a statement. We believe this independent review -- done at the FBI's request -- will help strengthen the law enforcement and national security community's scientific and analytical capabilities in future investigations.

Using tools such as high-throughput, 'next generation' DNA sequencing could have strengthened or weakened the association between spores found in the mailed letters and spores from RMR-1029, said David A. Relman, vice chair of the committee and professor at the Stanford University School of Medicine, in a statement. Such new technology will be important to similar investigations in the future.

Immediately after the attacks, the FBI established a repository of the Ames strain of anthrax, which could be used to compare the samples from the mailings. But the NRC report found that there were problems with the repository that affect the strength of the conclusions made using it.

The report says it takes no position on the guilt or innocence of Ivins, or anyone else. In addition the NRC did not get to review classified material.

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