The radio frequency interference that disabled communication between air traffic controllers at Tan Son Nhat International Airport and many flights on Tuesday might have stemmed from inside the airport, an official said Wednesday. While the possibility that an external frequency source interfered with the frequency air traffic controllers use to communicate with airplanes was ruled out, its true source remains a mystery, Nguyen Van Thu, deputy head of the Authority of Radio Frequency Management (ARFM), told Tuoi Tre (Youth) newspaper. Ho Chi Minh City’s Tan Son Nhat, Vietnam’s busiest airport, called for help from the ARFM immediately after the 18-minute incident occurred on Tuesday morning, affecting about ten flights that were heading for, or about to land, at the airport. The agency is still cooperating with relevant agencies to identify the cause of the incident, but Thu asserted that there was no external interference. It will take a lot of time to determine the cause as it requires multiple analyses, checks and tests, he admitted. Thu added that it is not an unprecedented incident. “We once handled a case in which the air traffic control frequency was interfered with by an outside source, which affected the navigation radar system,” he said. The Civil Aviation Authority of Vietnam (CAAV), meanwhile, also ruled out the possibility that the incident was caused by a broken piece of equipment in the airport’s air traffic control tower. “We have yet to be able to arrive at the final conclusion on the case,” CAAV chief Lai Xuan Thanh admitted to Tuoi Tre. Besides the ARFM and the CAAV, the Vietnam Air Traffic Management Corporation also sent a team to join the effort to find the cause of the interference. Doctor Tran Van Su, dean of the electronics and telecommunications department of the Ho Chi Minh City University of Technology, is pessimistic that the search will be successful. “If the interference was brought about by a certain kind of device that has been turned off and stopped transmitting a radio frequency, it will never be found,” he said. Six long-haul flights were heading for Tan Son Nhat, whereas three others were preparing to land, collectively carrying nearly 2,000 passengers, at that time, according to the Vietnam Air Traffic Management Corporation. These flights were then asked to wait, but one of them had to land at a different airport. Tan Son Nhat is Vietnam’s largest airport in terms of capacity, now handling more than 20 million passengers a year. On November 20 last year, the airport suffered an hour-long blackout in an unprecedented blow to the Vietnamese aviation industry. The incident disabled the radar system that controls air traffic at the terminal and affected as many as 92 flights.