  We show that the energy gap induced by ferromagnetically aligned magnetic
impurities on the surface of a topological insulator can be filled, due to
scattering off the non-magnetic potential of the impurities. In both a
continuum surface model and a three-dimensional tight-binding lattice model, we
find that the energy gap disappears already at weak potential scattering as
impurity resonances add spectral weight at the Dirac point. This can help
explain seemingly contradictory experimental results as to the existence of a
gap.
