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 and the Reuven Ramaty High Energy Solar Spectroscopic Imager, respectively. By combining the microwave and HXR imaging spectroscopy observations with multifilter extreme ultraviolet and soft X-ray imaging data, we derive the coronal magnetic field and energetic electron distribution of the source over a broad energy range from $łt$10 keV up to ensuremath∼MeV during the early impulsive phase of the flare. The source has a strong magnetic field of over 800 G. The best-fit electron distribution consists of a thermal core'' from ensuremath∼25 MK plasma. A nonthermal power-law ``tail'' joins the thermal core at ensuremath∼16 keV with a spectral index of ensuremath∼3.6, which breaks down at above ensuremath∼160 keV to $gt$6.0. Temporally resolved analysis suggests that the electron distribution above the break energy rapidly hardens with the spectral index decreasing from $gt$20 to ensuremath∼6.0 within 20 s, or less than ensuremath∼10 Alfvén crossing times in the source. These results provide strong support for the above- the-loop-top source as the primary site where an ongoing bulk acceleration of energetic electrons is taking place very early in the flare energy release.