  Nonthermal electrons accelerated in solar flares produce electromagnetic
emission in two distinct, highly complementary domains - hard X-rays (HXRs) and
microwaves (MWs). This paper reports MW imaging spectroscopy observations from
the Expanded Owens Valley Solar Array of an M1.2 flare that occurred on 2017
September 9, from which we deduce evolving coronal parameter maps. We analyze
these data jointly with the complementary Reuven Ramaty High-Energy Solar
Spectroscopic Imager HXR data to reveal the spatially-resolved evolution of the
nonthermal electrons in the flaring volume. We find that the high-energy
portion of the nonthermal electron distribution, responsible for the MW
emission, displays a much more prominent evolution (in the form of strong
spectral hardening) than the low-energy portion, responsible for the HXR
emission. We show that the revealed trends are consistent with a single
electron population evolving according to a simplified trap-plus-precipitation
model with sustained injection/acceleration of nonthermal electrons, which
produces a double-powerlaw with steadily increasing break energy.
