Office workers, tourists and locals were strolling through Seoul’s bustling Gwanghwamun Square on Wednesday afternoon when the unmistakable sound of helicopters filled the air.

The four helicopters, identified by weapons expert Joost Olliemans as ROKA AH-64E Apache Guardians and UH-60P Black Hawks, made multiple passes at a low height at roughly the level of the upper floors of the nearby Seoul Finance Center, Channel A and Kyobo buildings.