  In the present paper we investigate pi0 pi0 and pi(+/-)pi0 photoproduction
off complex nuclei at incident beam energies of 400-460 MeV. Simulations of two
pion photoproduction on protons and nuclei are performed by means of a
semi-classical BUU transport model including a full coupled-channel treatment
of the final state interactions. Elastic scattering of the final state pions
with the nucleons in the surrounding nuclear medium is found to yield a
downward shift of the pi pi invariant mass distribution. We show that the
target mass dependence of the pi0 pi0 invariant mass spectrum as measured by
the TAPS collaboration can be explained without introducing medium effects
beyond absorption and quasi-elastic scattering of the final state particles. On
the other hand, we find considerable discrepancies with the data in the
pi(+/-)pi0 channel, which are not understood.
