  We have conducted a comprehensive mid-IR spectroscopic investigation of 67
Low Ionization Nuclear Emission Line Regions (LINERs) using archival
observations from the high resolution modules of the Infrared Spectrograph on
board the Spitzer Space Telescope. Using the [NeV] 14 and 24um lines as active
galactic nuclei (AGN) diagnostics, we detect active black holes in 39% of the
galaxies in our sample, many of which show no signs of activity in either the
optical or X-ray bands. In particular, a detailed comparison of
multi-wavelength diagnostics shows that optical studies fail to detect AGN in
galaxies with large far-IR luminosities. These observations emphasize that the
nuclear power source in a large percentage of LINERs is obscured in the
optical. Indeed, the majority of LINERs show mid-IR [NeV]14/[NeV]24um flux
ratios well below the theoretical low-density limit, suggesting that there is
substantial extinction toward even the [NeV]-emitting region . Combining
optical, X-ray, and mid-IR diagnostics, we find an AGN detection rate in LINERs
of 74%, higher than previously reported statistics of the fraction of LINERs
hosting AGN. The [NeV]24um /[OIV]26um mid-IR line flux ratio in "AGN-LINERs" is
similar to that of standard AGN, suggesting that the spectral energy
distribution (SED) of the intrinsic optical/UV continuum is similar in the two.
This result is in contrast to previous suggestions of a UV deficit in the
intrinsic broadband continuum emission in AGN-LINERs. Consistent with our
finding of extinction to the [NeV]-emitting region, we propose that extinction
may also be responsible for the observed optical/UV deficit seen in at least
some AGN-LINERs.
