  We investigate in detail the 21 May 2004 flare using simultaneous
observations of the Nobeyama Radioheliograph, Nobeyama Radiopolarimeters,
Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and Solar and
Heliospheric Observatory (SOHO). The flare images in different spectral ranges
reveal the presence of a well-defined single flaring loop in this event. We
have simulated the gyrosynchrotron microwave emission using the recently
developed interactive IDL tool GX Simulator. By comparing the simulation
results with the observations, we have deduced the spatial and spectral
properties of the non-thermal electron distribution. The microwave emission has
been found to be produced by the high-energy electrons ($>100$ keV) with a
relatively hard spectrum ($\delta\simeq 2$); the electrons were strongly
concentrated near the loop top. At the same time, the number of high-energy
electrons near the footpoints was too low to be detected in the RHESSI images
and spatially unresolved data. The SOHO Extreme-ultraviolet Imaging Telescope
images and the low-frequency microwave spectra suggest the presence of an
extended "envelope" of the loop with lower magnetic field. Most likely, the
energetic electron distribution in the considered flare reflects the localized
(near the loop top) particle acceleration (injection) process accompanied by
trapping and scattering.
