  We report the detection of the 6.4 keV Iron K{\alpha} emission line in two
infrared-luminous, massive, star-forming BzK galaxies at z = 2.578 and z = 2.90
in the CDF-S. The Chandra 4 Ms spectra of BzK4892 and BzK8608 show a reflection
dominated continuum with strong Iron lines, with rest-frame equivalent widths
EW-2.3 keV and 1.2 keV, respectively, demonstrating Compton thick obscuration
of the central AGN. For BzK8608 the line identification closely matches the
existing photometric redshift derived from the stellar emission. We use the
observed luminosities of the Iron K{\alpha} line, of the rest-frame mid-IR
continuum and of the UV rest-frame narrow emission lines to infer intrinsic
L(2-10 keV)>1e44 erg/s, about 1.0-2.5 dex larger than the observed ones, hence
confirming the presence of an absorber with N(H) > 1e24 cm-2. The two BzK
galaxies have stellar masses of 5e10 M\odot and, based on VLA 1.4 GHz and submm
870{\mu}m observations, they appear to host vigorous starburst activity with
SFR - 300-700 M\odot/yr that is also optically thick. We estimate that the AGN
might also conceivably account for an important fraction of the bolometric
far-IR emission of the galaxies. The implied volume density of Compton thick
(CT) AGN with L(2-10 keV)>1e44 erg/s is in agreement with predictions from
X-ray background synthesis models. These sources provide one of the first
clearcut observations of the long-sought phase of simultaneous, heavily
obscured quasar and star formation activity, predicted by models of massive
galaxy evolution at high redshifts.
