  Supernova remnants (SNRs) are believed to be the main sources of Galactic
cosmic rays. Molecular clouds associated with SNRs can produce gamma-ray
emission through the interaction of accelerated particles with the concentrated
gas. The middle aged SNR W28, for its associated system of dense molecular
clouds, provides an excellent opportunity to test this hypothesis. We present
the AGILE/GRID observations of SNR W28, and compare them with observations at
other wavelengths (TeV and 12CO J=1-->0 molecular line emission). The gamma-ray
flux detected by AGILE from the dominant source associated with W28 is (14 +-
5) 10^-8 ph cm^-2 s^-1 for E > 400 MeV. This source is positionally well
correlated with the TeV emission observed by the HESS telescope. The local
variations of the GeV to TeV flux ratio suggest a difference between the CR
spectra of the north-west and south molecular cloud complexes. A model based on
a hadronic-induced interaction and diffusion with two molecular clouds at
different distances from the W28 shell can explain both the morphological and
spectral features observed by AGILE in the MeV-GeV energy range and by the HESS
telescope in the TeV energy range. The combined set of AGILE and H.E.S.S. data
strongly support a hadronic model for the gamma-ray production in W28.
