  We present ground-based infrared K- (2-2.5 micron) and L-band (2.8-4.1
micron) spectroscopy, as well as interferometric observations at 3mm, for the
individual merging components (A, B, and C) of the luminous infrared galaxy Arp
299. We investigate the presence and location of the putative buried active
galactic nucleus (AGN) inferred from previous X-ray observations at E > 10 keV.
Our sub-arcsec-resolution infrared spectra clearly reveal that the putative
buried AGN resides in the nucleus B1 (a subcomponent of B), based on a very low
equivalent width of 3.3 micron polycyclic aromatic hydrocarbon emission, a weak
2.3 micron CO absorption feature, and a large time variation of the K- and
L-band continuum fluxes. In component C, we find strong 3.1 micron ice
absorption at L and weak 2.3 micron CO absorption at K, as expected in a buried
AGN; however, a centrally concentrated young super star cluster is an
alternative possibility because of the modest infrared luminosity and
non-galaxy-nucleus nature of this component. The infrared K- and L-band spectra
of the infrared brightest nucleus, A, are typical of a normal starburst with no
explicit AGN signatures. Our interferometric observations simultaneously obtain
HCN (J=1-0) and HCO+ (J=1-0) emission lines with 4 arcsec resolution, and we
find the HCN to HCO+ brightness-temperature ratios to be as low as those found
in starburst nuclei in all the major merging components of Arp 299. The low
ratio even in the AGN-hosting nucleus B may be due to the presence of a large
amount of high-density molecular gas whose chemistry is dominated by coexisting
starbursts and/or shocks, rather than by the central strong X-ray-emitting AGN.
