  We present the results of L-band spectroscopical observations of local bright
ULIRGs, performed with ISAAC at the VLT. We discuss the L-band spectral
features of seven single sources, and the statistical properties of a complete
sample of 15 sources obtained combining our observations with other published
3-4 micron spectra. Our main results are: 1. When a spectral indicator
suggesting the presence of an AGN (low equivalent width of the 3.3 micron
emission line, steep lambda-f_lambda spectrum, presence of an absorption
feature at 3.4 micron) is found, the AGN is always confirmed by independent
analysis at other wavelengths. Conversely, in all known AGNs at least one of
the above indicators is present. 2. Two new diagnostic diagrams are proposed
combining the above indicators, in which starbursts and AGNs are clearly and
completely separated. 3. The above diagnostic techniques are possible with
spectra of relatively low quality, which can be obtained for several tens of
ULIRGs with currently available telescopes. This makes L-band spectroscopy the
current best tool to disentangle AGNs and starbursts contributions in ULIRGs.
4. The L-band properties of ULIRGs are heterogeneous. However, we show that all
the spectral differences among ULIRGs can be reproduced starting from pure
intrinsic AGN and SB spectra and varying two parameters: the amount of dust
extinction of the AGN component, and the relative AGN/SB contribution to the
bolometric luminosity. 5. Using the above decomposition model, we show that
AGNs in ULIRGs have a low dust-to-gas ratio and a dust extinction curve
different from Galactic. 6. The estimate of the contribution of AGNs in a
complete sample show that AGNs are hosted by ~2/3 of ULIRGs, but their
energetic contribution is relevant only in ~20% of the sample.
