UAS pilot reported an engine failure during cruise; resulting in an off airport landing without major damage to the medium UAS.

Narrative

Medium UAS operator with Commercial Pilot License with Instrument Rating.Medium UAS Engine Out Event.8nm South of ZZZ.Day 0; 2025 @ XI49.I was the RPIC for the flight. Throughout the entire flight duration of over 4.5 hours; everything was nominal until we were finishing up the last leg of our final field collect over Location A. At this point; approximately XA:53:00 Z; without any leading indicators of warning; the RPM went zero; and I took immediate action to turn the aircraft away from any property; assets; and people to safely pick a landing spot to execute a landing plan over a dirt field. We were at ~3;300 ft. AGL when the engine stopped running. From the point of view of the tail camera; the landing approach and landing looked to be clear of any obstacles. When the aircraft was recovered shortly after; ~1.5 hours after touchdown; the entire landing area was inspected; and no visual property damage; fire; or any injuries were seen on location. The aircraft was intact and was then transported back to the airfield to further investigate the root cause of the engine shutdown.Still under inspection to figure out the cause of the engine shutting off. ALL UAS operations are currently stopped until we can figure out the cause of the engine. Note that; the UAS engine can only be started from the ground.