Officials have concluded that over a dozen high-flying aircraft briefly disappeared from air traffic control screens over central Europe last year because the planes' radar systems were overloaded.

The incidents on June 5 and 10, 2014, forced air traffic controllers in Austria and neighboring countries to take emergency measures, guiding pilots by voice communication to prevent midair collisions.

The European Aviation Safety Agency says in a report that the source of the transmissions which overloaded the planes' transponders was in a 69-mile radius to the east of the Czech capital Prague.

The confidential report, obtained Friday by The Associated Press, notes that five surveillance systems manufacturers are based in the area and it was "highly probable" that tests conducted by the companies were the source of the interference.