  We present low resolution Spitzer-IRS spectra of 40 ETGs, selected from a
sample of 65 ETGs showing emission lines in their optical spectra. We
homogeneously extract the mid-infrared (MIR) spectra, and after the proper
subtraction of a "passive" ETG template, we derive the intensity of the ionic
and molecular lines and of the polycyclic aromatic hydrocarbon emission
features. We use MIR diagnostic diagrams to investigate the powering mechanisms
of the ionized gas. The mid-infrared spectra of early-type galaxies show a
variety of spectral characteristics. We empirically sub-divide the sample into
five classes of spectra with common characteristics. Class-0, accounting for
20% of the sample, are purely passive ETGs with neither emission lines nor PAH
features. Class-1 show emission lines but no PAH features, and account for
17.5% of the sample. Class-2, in which 50% of the ETGs are found, as well as
having emission lines, show PAH features with unusual ratios, e.g. 7.7
{\mu}m/11.3 {\mu}m \leq 2.3. Class-3 objects have emission lines and PAH
features with ratios typical of star-forming galaxies. 7.5% of objects fall in
this class, likely to be objects in a starburst/post-starburst regime. Class-4,
containing only 5% of the ETGs, is dominated by a hot dust continuum. The
diagnostic diagram [Ne III]15.55{\mu}m/[Ne II]12.8{\mu}m vs. [S
III]33.48{\mu}m/[Si II]34.82{\mu}m, is used to investigate the different
mechanisms ionizing the gas. If we exclude NGC 3258 where a starburst seems
present, most of our ETGs contain gas ionized via either AGN-like or shock
phenomena, or both. Most of the spectra in the present sample are classified as
LINERs in the optical window. The proposed MIR spectral classes show
unambiguously the manifold of the physical processes and ionization mechanisms,
from star formation, low level AGN activity, to shocks, present in LINER
nuclei.
