Building on a previously developed virtual-source model for loss-less dielectric slabs attached to metallic planes, we present a fast focused raster scanned spot continuous-wave (CW) method to find the thickness, dielectric permittivity, and loss-tangent of lossy dielectric slabs that could potentially be explosive threats. A library of reflection responses for objects with different sizes and electric properties is precomputed, and an inversion algorithm incorporates the measured data to predict the object features. It is shown that this characterization is possible even using only a single working frequency. The method is validated empirically by comparing to the synthetic reflection of the millimeter-wave imaging radar.