  Nonthermal sources located above bright flare arcades, referred to as the
"above-the-loop-top" sources, have been often suggested as the primary electron
acceleration site in major solar flares. The X8.2 limb flare on 2017 September
10 features such an above-the-loop-top source, which was observed in both
microwaves and hard X-rays (HXRs) by the Expanded Owens Valley Solar Array
(EOVSA) and the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI),
respectively. By combining the microwave and HXR imaging spectroscopy
observations with multi-filter extreme ultraviolet and soft X-ray imaging data,
we derive the energetic electron distribution of this source over a broad
energy range from $<$10 keV up to $\sim$MeV during the early impulsive phase of
the flare. The best-fit electron distribution consists of a thermal "core" from
$\sim$25 MK plasma. Meanwhile, a nonthermal power-law "tail" joins the thermal
core at $\sim$16 keV with a spectral index of $\sim$3.6, which breaks down at
above $\sim$160 keV to $>$6.0. In addition, temporally resolved analysis
suggests that the electron distribution above the break energy rapidly hardens
with the spectral index decreasing from $>$20 to $\sim$6.0 within 20 s, or less
than $\sim$10 Alfv\'{e}n crossing times in the source. These results provide
strong support for the above-the-loop-top source as the primary site where an
on-going bulk acceleration of energetic electrons is taking place very early in
the flare energy release.
