  We report on the source of greater than 300 MeV protons during the
SOL2014-09-01 sustained gamma-ray emission (SGRE) event based on
multi-wavelength data from a wide array of space- and ground-based instruments.
Based on the eruption geometry we provide concrete explanation for the
spatially and temporally extended {\gamma}-ray emission from the eruption. We
show that the associated flux rope is of low inclination (roughly oriented in
the east-west direction), which enables the associated shock to extend to the
frontside. We compare the centroid of the SGRE source with the location of the
flux rope leg to infer that the high-energy protons must be precipitating
between the flux rope leg and the shock front. The durations of the
SOL2014-09-01 SGRE event and the type II radio burst agree with the linear
relationship between these parameters obtained for other SGRE events with
duration exceeding 3 hrs. The fluence spectrum of the SEP event is very hard,
indicating the presence of high-energy (GeV) particles in this event. This is
further confirmed by the presence of an energetic coronal mass ejection (CME)
with a speed more than 2000 km/s, similar to those in ground level enhancement
(GLE) events. The type II radio burst had emission components from metric to
kilometric wavelengths as in events associated with GLE events. All these
factors indicate that the high-energy particles from the shock were in
sufficient numbers needed for the production of {\gamma}-rays via neutral pion
decay.
