Between May and December 1946, the Swedish Defence Staff logged approximately 2,000 sightings of cigar-shaped objects (the so-called 'ghost rockets', spökraketer) over Scandinavia. On October 10, 1946, the Defence Staff publicly stated that some 200 of the observations could not be explained as natural phenomena, Swedish aircraft, or imagination.

Försvarets Forskningsanstalt (FOA, the Swedish National Defence Research Institute) examined recovered debris fragments and radar-confirmed roughly 200 of the events. The investigation was classified at the time; files held at the Riksarkivet (Swedish National Archives) were declassified in 1984. The episode was a NATO-aligned intelligence concern in the early postwar period and remains the largest-scale national investigation of anomalous aerial objects in Swedish history.