  Differential cross sections of the reactions $(\gamma,\pi^\circ\pi^\circ)$
and $(\gamma,\pi^\circ\pi^++\pi^\circ\pi^-)$ have been measured for several
nuclei ($^1$H,$^{12}$C, and $^{\rm nat}$Pb) at an incident-photon energy of
$E_{\gamma}$=400-460 MeV at the tagged-photon facility at MAMI-B using the TAPS
spectrometer. A significant nuclear-mass dependence of the $\pi\pi$
invariant-mass distribution is found in the $\pi^\circ\pi^\circ$ channel. This
dependence is not observed in the $\pi^\circ\pi^{+/-}$ channel and is
consistent with an in-medium modification of the $\pi\pi$ interaction in the
$I$=$J$=0 channel. The data are compared to $\pi$-induced measurements and to
calculations within a chiral-unitary approach.
