Precursors provide early warning.

Certain selected chemicals associated with terrorist activities are too unstable to be prepared in final form. These chemicals are often prepared as precursor components, to be combined at a time immediately preceding the detonation. One example is a liquid explosive, which usually requires an oxidizer, an energy source, and a chemical or physical mechanism to combine the other components. Detection of the oxidizer (e.g. H₂O₂) or the energy source (e.g., nitromethane) is often possible, but must be performed in a short time interval (e.g., 5–15 seconds) and in an environment with a very small concentration (e.g.,1–100 ppm), because the target chemical(s) is carried in a sealed container.