  We use Herschel-PACS far-infrared data, combined with previous multi-band
information and mid-IR spectra, to properly account for the presence of an
active nucleus and constrain its energetic contribution in luminous infrared
(IR) sources at z\sim2. The sample is composed of 24 sources in the GOODS-South
field, with typical IR luminosity of 10^{12} Lo. Data from the 4 Ms Chandra
X-ray imaging in this field are also used to identify and characterize AGN
emission. We reproduce the observed spectral energy distribution (SED),
decomposed into a host-galaxy and an AGN component. A smooth-torus model for
circum-nuclear dust is used to account for the direct and re-processed
contribution from the AGN. We confirm that galaxies with typical
L_{8-1000um}\sim10^{12}Lo at z\sim2 are powered predominantly by
star-formation. An AGN component is present in nine objects (\sim35% of the
sample) at the 3sigma confidence level, but its contribution to the 8-1000 um
emission accounts for only \sim5% of the energy budget. The AGN contribution
rises to \sim23% over the 5-30 um range (in agreement with Spitzer IRS results)
and to \sim60% over the narrow 2-6 um range. The presence of an AGN is
confirmed by X-ray data for 3 (out of nine) sources, with X-ray spectral
analysis indicating the presence of significant absorption, i.e.
NH\sim10^{23}-10^{24} cm^{-2}. An additional source shows indications of
obscured AGN emission from X-ray data. The comparison between the
mid-IR--derived X-ray luminosities and those obtained from X-ray data suggests
that obscuration is likely present also in the remaining six sources that
harbour an AGN according to the SED-fitting analysis.
