The joint military-police task force investigating an alleged civilian drone deployment to North Korea summoned a graduate school student, who claims to have flown the drones, for a third round of questioning.

Investigators said Tuesday that they began interrogating the main suspect in the case, identified by the surname Oh, on charges of violating the Aviation Safety Act.

The team also summoned an executive of a drone company co-founded by Oh, identified by the surname Kim.

Oh, and another suspect accused of manufacturing the drones, identified by the surname Jang, had each been questioned twice.

Oh earlier told a local media outlet that he flew the drones three times to verify radiation levels at a North Korean uranium facility.

The drone probe has expanded to include allegations that Oh received funding from the Korea Defense Intelligence Command after he was named a cooperative partner in maneuvering operations.