We have measured the electron energy dependence for production of a great variety of anion fragments, induced by resonant attachment of subionization electrons to thymine (T) and cytosine (C) within femto-second time scales. At the lowest electron energies we also observe stable molecular anions of these bases, viz., T- and C-. Our measurements suggest that this resonant mechanism may relate to critical damage of irradiated cellular DNA by subionization electrons prior to thermalization.