  The small angular scale fluctuations of the (on large scale) isotropic
gamma-ray background (IGRB) carry information about the presence of unresolved
source classes. A guaranteed contribution to the IGRB is expected from the
unresolved gamma-ray AGN while other extragalactic sources, Galactic gamma-ray
source populations and dark matter Galactic and extragalactic structures (and
sub-structures) are candidate contributors. The IGRB was measured with
unprecedented precision by the Large Area Telescope (LAT) on-board of the Fermi
gamma-ray observatory, and these data were used for measuring the IGRB angular
power spectrum (APS). Detailed Monte Carlo simulations of Fermi-LAT all-sky
observations were performed to provide a reference against which to compare the
results obtained for the real data set. The Monte Carlo simulations are also a
method for performing those detailed studies of the APS contributions of single
source populations, which are required in order to identify the actual IGRB
contributors. We present preliminary results of an anisotropy search in the
IGRB. At angular scales <2deg (e.g. above multipole 155), angular power above
the photon noise level is detected, at energies between 1 and 10 GeV in each
energy bin, with statistical significance between 7.2 and 4.1 sigmas. The
energy not dependence of the fluctuation anisotropy is pointing to the presence
of one or more unclustered source populations, while the energy dependence of
the intensity anisotropy is consistent with source populations having average
photon index 2.40\pm0.07.
