  The efficiency of mid-infrared selection methods for finding obscured AGN is
investigated using data in the \chandra Deep Field North. It is shown that
samples of AGN candidates compiled on the basis of mid-infrared colours only
suffer substantial contamination from normal galaxies. X-ray stacking analysis
reveals a soft mean X-ray spectrum for these sources, consistent with
Gamma~2.1. This suggests that star-forming galaxies and not obscured AGN
dominate the stacked signal. In contrast AGN selection methods that combine
mid-infrared with optical criteria are more successful in finding heavily
obscured AGN candidates. A method similar to the one proposed by Fiore et al.
(2008) is adopted to select extremely red objects (R-[3.6]>3.7mag) with high
24micron to optical flux ratio (f_24/f_R>1000). About 80% of these sources are
not detected at X-ray wavelengths. Stacking the X-ray photons at the positions
of these sources shows a flat mean X-ray spectrum (Gamma~0.8), which suggests
Compton-thick sources, low-luminosity and moderately obscured (N_H~8e22) AGN,
or a combination of the two. The mid-infrared colours and luminosities of these
sources are consistent with ULIRGs at z~2, while HST/ACS images, available for
the optically brighter of these sources, show disturbed optical morphologies in
many of them. The evidence above suggests that this population includes systems
in the process of formation.
