The bombs used in an apparent terror attack in New York City this weekend reportedly (see for example) contained residue of a material similar to Tannerite®, a mixture of ammonium nitrate and aluminum powder. This first use of this material in a successful improvised explosive device (IED) demonstrate the problems that will be faced by the DHS contracted study on reducing the threat of IED by limiting access to explosive precursor chemicals.
Tannerite explosive targets are currently available without any legal controls. Tannerite-like explosives can be easily made by anyone with access to ammonium nitrate and aluminum powder, neither of which currently have any restrictions on their sale. In fact, ammonium nitrate can be found in many commercially available cold packs and aluminum powder can be made by anyone with access to aluminum metal.
Congress tried to get DHS to regulate the sale of large quantities of ammonium nitrate, but coming up with cost effective regulations that would make access to the large quantities that were used in the Oklahoma Federal Building bombing has proven to be impossible. This is the reason that DHS has contracted with the Academies of Science to study possible controls on the supplies of IED precursors.
While making large-scale IEDs (truck bombs) may be possible to limit by controlling access to ammonium nitrate, the large number of chemicals that can be combined to make small-scale IEDs (pressure cooker bombs) ensure that the government will never be able to shut off the flow of commercial chemicals into these types of devices.
To feasibly restrict access to precursors to IEDs someone is going to have to decide where to set the size limit on IEDs; the size of an explosive device that makes it worthwhile to impose costly controls on the sale and distribution of the chemicals involved. We are not going to be able to stop the making of small IEDs through chemical sale controls without paralyzing legitimate chemical commerce.