  A field-theoretical description of the photoproduction of two pions off the
nucleon is presented that applies to real as well as virtual photons in the
one-photon approximation. The Lorentz-covariant theory is complete at the level
of all explicit Faddeev-type three-body final-state mechanisms of dressed
interacting hadrons, including those of the nonlinear Dyson-Schwinger type. All
electromagnetic currents are constructed to satisfy their respective
(generalized) Ward-Takahashi identities and thus satisfy local gauge invariance
as a matter of course. The Faddeev-type ordering structure results in a natural
expansion of the full two-pion photoproduction current $\Mpp^\mu$ in terms of
multiple loops that preserve gauge invariance order by order in the number of
loops, which in turn lends itself naturally to practical applications of
increasing sophistication with increasing number of loops.
