AGNs have recently been recognized as contributing in significant numbers to the radio source counts at sub-mJy flux densities, but still controversial are their physical and evolutionary properties. One of the key unresolved issues is whether, as a function of cosmic epoch, the low-power AGNs are more related to efficiently accreting systems (mostly radio-quiet) or to systems with very low accretion rates (mostly radio-loud).

We are currently exploiting the deep radio/optical/IR information available for the Spitzer First Look Survey (FLS) to investigate the physical properties of faint radio-selected AGNs. Such a study is to be included in a more general analysis of several radio deep fields with multi-wavelength information available. In particular we exploit here the well-know radio/infrared correlation, holding for star-forming galaxies, to efficiently select out radio sources triggered by AGNs. Preliminary results clearly shows that the large majority of low power AGNs are radio-loud, possibly associated to low accretion rate systems. Very interestingly, however, we have here a first direct evidence of radio-quiet AGNs being present at sub-mJy flux levels. Such a result supports the idea that radio-quiet AGNs are not necessarily radio silent. Under such an assumption, their radio properties (1.4 GHz Luminosity $\leq 10^{24}$ W/Hz; steep radio spectrum) are fully consistent with those expected from recent modeling of the radio-quiet AGN population, and, as expected, they are associated to galaxies showing AGN-like emission lines in their optical spectra.