Anthrax Detection

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"Anthrax Detection"

Anthrax Contaminated Letters Sent to U.S. Capitol

Government document

By: The United States Government Accountability Office

Date: April 5, 2005

Source: "Anthrax Detection: Agencies Need to Validate Sampling Activities in Order to Increase Confidence in Negative Results," as published by the United States Government Accountability Office.

About the Author: The United States Government Accountability Office is the investigative arm of congress that examines government efficiency by evaluating federal programs, auditing federal expenditures, and issuing legal opinions.


Shortly after the terrorists attacks of September 11, 2001, anthrax laced letters began appearing in the mailboxes of some U.S. senators, major U.S. news media personnel, and a few other individuals (who initially appeared as random receivers). At first, U.S. government and media reports asserted that these letters were connected with the al-Qaeda attacks on the United States, but the investigation of the anthrax source and initial mail drop of the letters substantiated the claim that the contaminated mail originated within the United States and from U.S. laboratories.

Anthrax is a bacterial disease that usually is transmitted from plant eating animals, and human infections in the United States are rare. Most commonly these infections occur from occupational hazards such as working in laboratories where the bacteria is created or handling infection-prone animals. Furthermore, a person can acquire anthrax by eating undercooked contaminated meat, having the bacteria enter the body through an open wound, or by inhaling airborne spores. Inhaling the spores is the most efficient way for anthrax to spread (especially among humans) because the spores can travel through the air, land on papers, and linger for a significant amount of time. While anthrax can be treated, victims can die if medical treatment is not sought in a timely manner. Anthrax is considered a weaponizable biological agent because of its airborne qualities and lethal implications.

The United States had considered the threat of anthrax long before the October 2001 letters that targeted media outlets and shut down the Hart Senate building in Washington, D.C. One Federal Bureau of Investigations (FBI) report states that in 2000, it dealt with 250 suspected cases of weapons of mass destruction and biological weapons, with about 200 of those cases concerning anthrax. These instances were deemed hoaxes, but the FBI initiated response programs to handle possible future anthrax threats.

Even though federal agencies had developed new strategic plans to deal with terrorism threats from biological agents, not all of the new counterterrorism measures had been put into place. The first anthrax letters began appearing a few weeks after the September 11 attacks. It was later disclosed that the letters were mailed on September 11. The first victims of anthrax were magazine publishers and postal workers, but this quickly changed when employees of the television network NBC New York tested positive for the bacteria. Then on October 15, 2001, letters sent to Senate majority leader Tom Daschle tested positive for anthrax, and on October 17, the U.S. capitol shut down (particularly the Hart Senate building) when thirty-one employees test positive. Most of the employees worked for Daschle. A similar letter was found addressed to Senator Patrick Leahy.

A federal investigation of the anthrax letter attacks immediately commenced. President George W. Bush labeled the anthrax letters as acts of terrorism. Initial reports connected the attacks to Bin Laden's al-Qaeda terrorist network, but FBI investigators quickly disclosed that al-Qaeda was most likely not responsible for the letters.

The 2001 anthrax letter attacks killed five people.



The first activity involved agencies' developing a sampling strategy, which included deciding how many samples to collect, where to collect them from, and what collection methods to use. The agencies primarily used a targeted strategy: They collected samples from specific areas considered more likely to be contaminated, based on judgments. Such judgments can be effective in some situations, for example, in determining (1) the source of contamination in a disease outbreak investigation or (2) whether a facility is contaminated when information on the source of potential contamination is definitive. However, in the case of a negative finding, when the source of potential contamination is not definitive, the basic question—Is this building contaminated?—will remain unanswered.

The targeted strategy the agencies used was reflected in their site-specific sampling activities. Sample sizes varied by facility and circumstances, increased over time, and excluded probability sampling. In the beginning, in each USPS facility, 23 samples were to be collected from specific areas relating to mail processing and up to 20 additional "discretionary" samples were to be collected, depending on the type and size of the facility. Later, USPS increased the number of samples required to a minimum of 55, with up to 10 additional discretionary samples for larger facilities. Consequently, the number of samples collected varied by facility, from a low of 4 to a high of 148. CDC's [Centers for Disease Control and Prevention] and EPA's [Environmental Protection Agency] site-specific strategies were primarily discretionary. The number of samples CDC collected varied by facility, ranging from a low of 4 to a high of 202. The number of samples EPA collected ranged from a low of 4 to a high of 71.

According to CDC, a targeted sampling strategy may be effective in detecting contamination in a facility when sufficient site-specific information exists to narrow down the locations in which the release and contamination are most likely to have occurred. CDC's assumptions for this strategy are that at the outset, (1) a scenario where all locations have an equal chance of being contaminated is generally the exception rather than the rule; (2) information collected about the event, combined with technical judgment about exposure pathways, can be used to identify locations where contamination is most likely to be found; (3) contamination levels of the highest public health concern can usually be detected using a variety of available methods, despite their limitations; and (4) there is important public health value in quickly identifying contaminated locations. However, these assumptions may not always apply. For example, there may be limitations in the available information that restrict the ability to reliably identify target locations. The method of contamination spread could conceivably be via a mechanism where there is an equal chance of any area being contaminated. Lastly, all results may be negative, which will lead to a requirement for additional testing, as was the case in Wallingford. This, in turn, will result in the loss of the critical time needed for public health intervention.

CDC and USPS officials said that they used a targeted strategy for several reasons, including limitations on how many samples could be collected and analyzed. They also said that in 2001 they lacked the data necessary to develop an initial sampling strategy that incorporated probability sampling. We disagree with this interpretation. Probability sampling is statistically based and does not depend solely on empirical criteria regarding the details of possible contamination.

We consider probability sampling to be a viable approach that would address not only the immediate public health needs but also the wider public health protection, infrastructure cleanup, and general environmental contamination issues. We recognize that in a major incident, the number of samples that may need to be collected and analyzed may challenge available laboratory resources. Accordingly, there is a need to develop innovative approaches to use sampling methods that can achieve wide-area coverage with a minimal number of individual samples to be analyzed. For example, high-efficiency particulate air (HEPA) vacuum techniques, in combination with other methods, appear to be one such approach that could achieve this. In addition, because of limited laboratory capacity, samples may need to be stored after collection for subsequent analysis, on a prioritized basis.

The situation in 2001 was unique, and the agencies were not fully prepared to deal with environmental contamination. In the future, if the agencies decide to use a targeted rather than a probability sampling strategy, they must recognize that they could lose a number of days if their targeted sampling produces negative test results. In this case, additional samples would need to be collected and analyzed, resulting in critical time, for public health interventions, being lost. This was so at the Wallingford postal facility in the fall of 2001, when about 3 weeks elapsed between the time the first sampling took place and the results of the fourth testing, which revealed positive results. Furthermore, about 5 months elapsed between the time of the first sampling event and the time anthrax was found in the Wallingford facility's high-bay area.

Therefore, in the future, strategies that include probability sampling need to be developed in order to provide statistical confidence in negative results. Further, even if information on all the performance characteristics of methods is not yet available, a probability sampling strategy could be developed from assumptions about the efficiency of some of the methods. And even if precise data are not available, a conservative, approximate number could be used for developing a sampling strategy. This would enable agencies and the public to have greater confidence in negative test results than was associated with the sampling strategy used in 2001.


In the aftermath of the anthrax letter attacks, the U.S. government strengthened security measures at all U.S. post offices. A new screening program was implemented for mail sent to Washington, D.C. These were just some of the precautions that legislators put into effect to possibly prevent future anthrax attacks using the postal system.

An elderly woman in Connecticut and a magazine employee in Florida both received anthrax letters because their mail was cross-contaminated during sorting and transport. These letters were processed through the same postal machines, held in the same barrels, and possibly transported in the same postal bags as the anthrax-laced letters sent to Washington, D.C. and New York.

There were limited copy-cat crimes on the heels of the anthrax letter attacks. However, none of the subsequent letters declaring that they contained biological agents tested positive for such agents. When Senator Daschle received a second letter months after the initial attack, investigators quickly deemed it a hoax when tests confirmed the powdery substance in it to be talcum powder.

By mid-2005, the sender of the anthrax letters had not yet been identified, but U.S. government and law enforcement agencies continued to investigate the matter.



Cole, Leonard A. The Anthrax Letters: A Medical Detective Story. Washington, D.C.: National Academy of Sciences, 2003.

Web sites "Leahy letter 'as lethal' as one sent to Daschle." <> (accessed July 6, 2005).