  Recent X-ray surveys have clearly demonstrated that a population of optically
dull, X-ray bright galaxies is emerging at 2-10 keV fluxes of the order of
10^{-14} erg cm^{-2} s^{-1}. Although they might constitute an important
fraction of the sources responsible for the hard X-ray background, their nature
is still unknown. With the aim to better understand the physical mechanisms
responsible for the observed properties, we have started an extensive program
of multiwavelength follow-up observations of hard X-ray, optically quiet
galaxies discovered with XMM-Newton. Here we report the results of what can be
considered the first example of this class of objects: CXOUJ031238.9-765134,
originally discovered by Chandra, and optically identified by Fiore et al.
(2000) with an apparently normal early-type galaxy at z=0.159, usually known as
"FIORE P3". The analysis of the broad-band energy distribution suggests the
presence of a heavily obscured active nucleus.
