  Based on detailed analysis of radio and X-ray observations of a flare on 2002
April 11 augmented by realistic 3D modeling, we have identified a radio
emission component produced directly at the flare acceleration region. This
acceleration region radio component has distinctly different (i) spectrum, (ii)
light curves, (iii) spatial location, and, thus, (iv) physical parameters from
those of the separately identified, trapped or precipitating electron
components. To derive evolution of physical parameters of the radio sources we
apply forward fitting of the radio spectrum time sequence with the
gyrosynchrotron source function with 5 to 6 free parameters. At the stage when
the contribution from the acceleration region dominates the radio spectrum, the
X-ray- and radio- derived electron energy spectral indices agree well with each
other. During this time the maximum energy of the accelerated electron spectrum
displays a monotonic increase with time from ~ 300 keV to ~ 2 MeV over roughly
one minute duration indicative of an acceleration process in the form of growth
of the power-law tail; the fast electron residence time in the acceleration
region is about 2 - 4 s, which is much longer than the time of flight and so
requires a strong diffusion mode there to inhibit free-streaming propagation.
The acceleration region has a relatively strong magnetic field, B ~ 120 G, and
a low thermal density, less than 2x10^9 cm^-3. These acceleration region
properties are consistent with a stochastic acceleration mechanism.
