A terahertz imaging system uses a terahertz light source to illuminate a target object with a series of short pulses of terahertz light. The pulses have a low average output power but a high peak output power, thereby enabling imaging of the suspicious object or person at a safe distance while avoiding injury to the person being imaged. For each pulse, the reflection of the light is imaged by a detector array. The terahertz light pulses are individually narrow band and the emitted pulse train is rapidly tuned in wavelength across a wide frequency range in the terahertz band. Within the terahertz frequency range, molecules have unique absorption spectra. The energy reflected from the illuminated threat object is measured for each of the discrete narrow frequency bands and compared to the absorption spectra of known dangerous materials to determine whether the threat object contains or carries a known dangerous material.