Hydrodynamical models have generally failed to describe interferometry radii measured at RHIC. In order to investigate this ``HBT puzzle'', we carry out a systematic study of HBT radii in ultrarelativistic heavy-ion collisions within a two-dimensional transport model. We compute the transverse radii $R_o$ and $R_s$ as functions of $p_t$ for various values of the Knudsen number, which measures the degree of thermalization in the system. For realistic values of the Knudsen number estimated from $v_2$ data, we obtain $R_o/R_s \simeq 1.2$, much closer to data than standard hydrodynamical results. Femtoscopic observables vary little with the degree of thermalization. Azimuthal oscillations of the radii in non central collisions do not provide a good probe of thermalization.