As part of the invasion of Norway by Nazi Germany on 9 April 1940, German Luftwaffe-aircraft landed at Fornebu. There was no attempt by the civilian airport authorities to hinder this, such as driving cars onto the runway, although several German aircraft collided with each other during the landing. A KLM aircraft had a scheduled service that morning, and the captain was ordered to leave the passengers, take the crew and return to Oslo. On 12 April, the airport was bombed by the British Royal Air Force. On 14 April, the KLM captain was granted permission to fly back to Amsterdam with the crew, albeit without any passengers. The German military used Fornebu heavily during the war, but it was never of any strategic importance, since it was located far from any battle zones. During the war, the airport officially remained owned by the municipality. By orders of the German authorities, the main north–south runway was expanded to , and all facilities not yet built were completed. However, during the war all other runways than the main north–south were taken out of use. At the north end of the runway, the Luftwaffe built several hangars and a prison camp. Prisoners were used to keep the runways free of snow during winter, by marching along them and stomping the snow down.