About 50 planes temporarily disappeared from radars in Austria, Germany, the Czech Republic, and Slovakia between June 5 and June 10, Austria's flight safety monitor said.
German and Czech air traffic control also reported brief disappearances.
Slovakia blamed the outages on planned military exercises.
"The disappearance of objects on radar screens was connected with a planned military exercise which took place in various parts of Europe...whose goal was the interruption of radio communication frequencies," the Slovak air traffic services said in a statement.
"This activity also caused the temporary disappearance of several targets on the radar display, while in the meantime the planes were in radio contact with air traffic controllers and continued in their flight normally."
Right after the problem with the radars was detected, the organizing party was made aware and the exercises were stopped, Slovakia's air traffic services added.
Slovakia did not mention a military force involved in the exercises, but Austrian media pointed towards NATO.
Austrian daily Kurier reported on June 7 that the first disappearance problems coincided with NATO conducting electronic warfare exercises in Hungary.
During the exercises NATO was reportedly using devices that can interfere with enemy radar, according to the Telegraph.
German air traffic control stated that it is trying to identify the cause of the outages.
"Planes disappeared from screens for a matter of seconds, here and there. The outages were sporadic and not grave...It must have been an external source of disruption," a spokesperson said.
Authorities said the outages did not pose any serious threat.
Comment: Or...the planes disappearing from radar could be a direct result of two X-Class solar flares that took place during the time frame in question. Data from the Space Weather Prediction Center illustrates just such an occurrence, as can be seen here with the GOES X-ray flux data.
X-Class solar flares have been observed to cause radio blackout due to the ionosphere's D-Region Absorption of said flares. Where was the affected region during one of these events? Europe, which can be seen here.
Currently the Sun's output is low relative to historical solar maximums and double X-Class flares could reasonably be seen as anomalous. Unless the flares were caused by Sun diving comets, which apparently was the case.
Watch this video for a quick rundown of how everything transpired in "real-time".