  We present the results of Spitzer IRS low-resolution infrared 5-35 micron
spectroscopy of nearby ultraluminous infrared galaxies (ULIRGs) at z < 0.15. We
focus on the search for the signatures of buried active galactic nuclei (AGNs)
in the complete sample of ULIRGs classified optically as non-Seyferts (LINERs
or HII-regions). In addition to polycyclic aromatic hydrocarbon (PAH) emission
features at 6.2 micron, 7.7 micron, and 11.3 micron, the conventional tool of
starburst-AGN separation, we use the optical depths of the 9.7 micron and 18
micron silicate dust absorption features to infer the geometry of energy
sources and dust at the nuclei of these ULIRGs, namely, whether the energy
sources are spatially well mixed with dust(a normal starburst) or are more
centrally concentrated than the dust (a buried AGN). Infrared spectra of at
least 30%, and possibly 50%, of the observed optical non-Seyfert ULIRGs are
naturally explained by emission consisting of (1) energetically insignificant,
modestly obscured (Av < 20-30 mag) PAH-emitting normal starbursts, and (2)
energetically dominant, highly dust-obscured, centrally concentrated energy
sources with no PAH emission. We interpret the latter component as a buried
AGN. The fraction of ULIRGs showing some buried AGN signatures is higher in
LINER ULIRGs than in HII-region ULIRGs. Most of the luminous buried AGN
candidates are found in ULIRGs with cool far-infrared colors. Where the
absorption-corrected intrinsic AGN luminosities are derivable with little
uncertainty, they are found to be of the order of 10^12Lsun, accounting for the
bulk of the ULIRGs' luminosities. The 5-35 micron spectroscopic starburst/AGN
classifications are generally consistent with our previous classifications
based on 3-4 micron spectra for the same sample.
