  We present the broad-band Spectral Energy Distributions (SEDs) of the largest
available highly (72%) complete spectroscopic sample of mid-infrared (MIR)
selected galaxies and AGN at intermediate redshift. The sample contains 203
extragalactic sources from the 15-micron survey in the ELAIS-SWIRE field S1,
all with measured spectroscopic redshift. Most of these sources have full
multi-wavelength coverage from the far-UV to the far-infrared and lie in the
redshift range 0.1<z<1.3. Due to its size, this sample allows us for the first
time to characterise the spectral properties of the sources responsible for the
strong evolution observed in the MIR. Based on SED-fitting technique we have
classified the MIR sources, identifying AGN signatures in about 50% of them.
This fraction is significantly higher than that derived from optical
spectroscopy (~29%) and is due in particular to the identification of AGN
activity in objects spectroscopically classified as galaxies. It is likely that
in most of our objects, the AGN is either obscured or of low-luminosity, and
thus it does not dominate the energetic output at any wavelength, except in the
MIR, showing up just in the range where the host galaxy SED has a minimum. The
fraction of AGN strongly depends on the flux density, with that derived through
the SED-fitting being about 20% at S(15)~0.5-1 mJy and gradually increasing up
to 100% at S(15)>10 mJy, while that obtained from optical spectroscopy never
being >30%, even at the higher flux densities. The results of this work will be
very useful for updating all the models aimed at interpreting the deep infrared
survey data and, in particular, for constraining the nature and the role of
dust-obscured systems in the intermediate/high-redshift Universe.
