The massless string spectrum on AdS_3 x S^3 from the supergroup

String theory on AdS_3 x S^3 is studied in the hybrid formulation. We give a detailed description of the PSL(2|2) supergroup WZW model that underlies the background with pure NS-NS flux, and determine the BRST-cohomology corresponding to the massless string states. The resulting spectrum is shown to match exactly with the expected supergravity answer, including the sectors with small KK momentum on the sphere.