XU (X for "unknown" and U for "undercover agent") was a clandestine intelligence organisation working on behalf of Allied powers in occupied Norway during World War II.
[1] During the early stages of the German occupation of Norway, Sand collaborated with an intelligence group led by major John Hagle and captain Eivind Hjelle.
Although XU was initially organized by a leadership with a military background, its further development relied heavily upon recruiting students from the University of Oslo.
As it grew, the group also included professionals around Norway, within railroads, police and so on, and collected maps and photos of German fortifications and forces.
By the end of the war XU had some 1,500 agents all over Norway and had developed a sophisticated courier system to the United Kingdom through neutral Sweden.
The government was criticized for waiting for so long, especially since many double agents, otherwise sworn to secrecy, were convicted as collaborators during the post-war treason trials.