Explosive Precursor Chemicals


Dr. Kirk Yeager warns us of possible CBRNe terrorist attacks using explosives.

Ever since the discovery of high explosives, bombers have subverted these materials to lash out against their perceived adversaries. Many explosive charges can be produced with seemingly innocuous materials. The Nashville Christmas Day Bomber used such chemicals to produce a charge of great devastation. This was done despite the best efforts of law enforcement to educate vendors of this lurking danger. Hopefully a lesson can be drawn from this attack which can be used to avoid a tragic repeat.

History of vehicle bombs

The United States is very fortunate in that it has not experienced a great number of vehicle bombs throughout its history. Prior to Nashville only four vehicle bombs of significant impact occurred.  A summary of these vehicle attacks is provided down below.

Vehicle Bomb Timeline 

The first attack over a century ago consisted of dynamite in a horse drawn carriage exploded in the financial district of Wall Street. In 1970 anti-war radicals detonated a ton of ANFO outside University of Wisconsin’s Sterling Hall, a building that housed a defense department research center, in protest of the Vietnam War. During the 1990’s two terrorist bombings, one international and one domestic, targeted the World Trade Center in New York City and the Murrah Federal building in Oklahoma. Unlike the previous attacks, the bomb in Nashville was not set off as an act of terrorism. Nor was it deployed to create a body count. It is only through the intention of the bomb builder that the explosion did not result in a much more significant loss of life.

Picture on the left: ©Wikimedia/New York World Trade Center

Chemical purchases

A review of the purchase history of the Nashville bomber was conducted to ascertain which explosive precursor chemicals (EPCs) he procured. Both big-box store purchases from Walmart and Sam’s Club as well as purchases from specialty chemical companies over a 10-year span were examined.  An overview of significant EPCs obtained, along with rough quantities, is provided in the following table.

EPC Purchases over 10 Year Period

Overall, the bomber purchased approximately 1550 pounds of oxidizers and 92 pounds of highly reactive fuels. Most terrorist explosives are based on simple combinations of these two ingredients.  Assuming an ideal ratio of fuel and oxidizer, these purchases could have produced an explosive charge ranging between 460-920 pounds.

The exact nature of the explosive charge produced remains unknown. Chemical analysis from the post-blast scene showed residues consistent with Nitrates, Perchlorates and Chlorates. Chemical analysis of residues collected from the bombers residence revealed the presence of Erythritol Tetranitrate (ETN) and Picric Acid (PA).

National academies studies

Weaponization of EPCs has been of keen interest in the U.S. since the mid-90’s. Both the World Trade Center and Oklahoma City bombers utilized commercial chemicals to produce their main charges. Following the bombing of the Murrah Federal Building, the United States government funded the National Research Council to examine approaches which could be taken to prevent such a tragedy from occurring again. One of the taskings undertaken in this study was to consider the “Feasibility and practicability of imposing controls on certain precursor chemicals”. Results from the study, to include a list of the most important EPCs to consider for controls, were released in 1998.

It is important to stress that this study, with a few exceptions, primarily focused on EPCs with the potential to produce large vehicle bombs. One of the criteria outlined for EPC selection explicitly stated, “the chemical should be reasonably adaptable (and available or accessible in amounts on the order of 100 pounds or more of explosive material) for use in making large bombs.”

In the years following the 1998 study the advent of the Internet allowed for the rapid transmission of explosive recipes to a wide population. As a result, a more diverse range of explosives began to emerge in bombings across the globe. To produce their charges, terrorists world-wide turned increasingly towards an expanding collection of EPCs.  

To address this growing problem, the Department of Homeland Security funded a study from the National Academies of Sciences, Engineering, and Medicine focused purely on EPCs. The author served on the committee selected to study the issue. As in the 1998 study, a list of EPC of concern was created, and the full analysis published.

A comparison of EPCs identified in both studies is provided in the following lines. The 1998 study identified only 11 EPCs of concern.  In 2018, a total of 28 EPCs of concern were identified. These were further divided into three groupings for ranking, with Group A deemed the most critical and Group C the least.

EPCs Identified for Potential Controls

Acetone (highlighted in yellow) was the only chemical identified in 1998 that was not carried forward into the 2018 list. Highlighted in blue are all the chemicals from the first study that were also included within the second.  As can be seen, the majority of the 1998 chemicals fell into the category of highest concern in 2018.

Nashville EPCs

During the quarter of a century (and two national studies) separating the Oklahoma City Bombing from Nashville, accessibility of EPCs has changed little. A simple crosswalk of EPCs identified as potential threats and purchases made by the Nashville bomber is provided down below.

EPCs from Nashville and National Studies

The EPCs listed are organized in the priority groups from the 2018 study. Red arrows indicate EPCs that were purchased by the Nashville bomber. Red boxes highlight chemicals identified of concern in 1998 and again in 2018. Blue arrows indicate instances where EPCs similar, but slightly different, to those listed in the table were procured.

“These same chemicals are available today to those whose intentions may not be as averse to violence as the Nashville Bomber. Absent some action to address this situation, another massive bombing awaits to fall on us as the sword onto Damocles”


The Nashville bomber procured 13 EPCs that were identified as materials of concern in 2018.  Five of these precursors were also listed in the 1998 study. Two of the EPCs called out in this study were purchased in hundreds of pounds. These same chemicals are available today to those whose intentions may not be as averse to violence as the Nashville Bomber. Absent some action to address this situation, another massive bombing awaits to fall on us as the sword onto Damocles.

About the Author:

Dr. Kirk Yeager is the FBI’s Chief Explosives Scientist, having served for 10 years as a Physical Scientist/Forensic Examiner for the FBI Laboratory’s Explosives Unit where he deployed as a bombing crime scene investigator to dozens of countries. Dr. Yeager has approximately 25 years of experience with improvised explosives and IEDs, has presented over 200 talks at international meetings and conferences, and has produced over 80 specialty publications on explosives and IEDs.

The views expressed in this article are those of the author and should not be interpreted as representing any official position of the FBI.

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