  We present the first results of the spectroscopy of distant, obscured AGN as
obtained with the ultra-deep (~3.3 Ms) XMM-Newton survey in the Chandra Deep
Field South (CDFS). One of the primary goals of the project is to characterize
the X-ray spectral properties of obscured and heavily obscured Compton-thick
AGN over the range of redhifts and luminosities that are relevant in terms of
their contribution to the X-ray background. The ultra-deep exposure, coupled
with the XMM detector's spectral throughput, allowed us to accumulate good
quality X-ray spectra for a large number of X-ray sources and, in particular,
for heavily obscured AGN at cosmological redshifts. Specifically we present the
X-ray spectral properties of two high-redshift - z= 1.53 and z=3.70 - sources.
The XMM spectra of both are very hard, with a strong iron Kalpha line at a
rest-frame energy of 6.4 keV. A reflection-dominated continuum provides the
best description of the X-ray spectrum of the z=1.53 source, while the
intrinsic continuum of the z=3.70 AGN is obscured by a large column N_H ~ 10^24
cm-2 of cold gas. Compton-thick absorption, or close to it, is unambiguously
detected in both sources. Interestingly, these sources would not be selected as
candidate Compton thick AGN by some multiwavelength selection criteria based on
the mid-infrared to optical and X-ray to optical flux ratios.
