This Friday, South Korean military inspectors will visit an army corps. Their job is to examine a recent incident in which troops almost fired down an unidentified aircraft near the heavily fortified border with North Korea. The Joint Chiefs of Staff (JCS) later confirmed the plane was a U.S. training drone that had been being part of a joint military exercise.

The JCS said the first sighting of the aircraft was in northern Gyeonggi province on Thursday. Normal operating procedures were used prior to proper identification of the drone. The event caused a big security scare, prompting military units to send out evacuation notices to residents and farmers in the Civilian Control Zone in Cheorwon and nearby areas.

At first, there was no way for the forces in South Korea to tell whether the plane was friendly or a possible North Korean threat. In response, the troops escalated the defense and were ready to go on the attack; they monitored its movement. Text messages were sent to local residents that requested shelter, but later clarification messages were sent, stating, “situation has been resolved.

The U.S. Forces Korea told local news media that the plane was from the U.S. Marine Corps and part of a joint training. The command is looking into the situation of the misidentification.

The incident kind of underscores the heightened sensitivity to air activity near the border at a moment when tensions are already high between the army. North Korea has demonstrated a hostile posture and, in fact, has consistently declined inter-Korean reconciliation initiatives, again and again.

The scare comes after another recent incident in which Pyeongtaek residents were warned for a few hours to evacuate their homes because of a white phosphorus incident near a U.S. military base, sparking calls from civic groups for an investigation and apology. The JCS readiness inspection office will conduct a detailed check on the JCS’s handling of the situation to make sure proper procedures were followed during the drone scare.