They monitored and analyzed the activity for two months using highly encrypted tools before teaming up with two high-tech security firms, which both confirmed that they thought spying equipment was being used in the area.

IMSI catchers are banned in most countries except for official use. In Norway, only the police and intelligence services have authorization to use them. They allow eavesdropping and tracking of mobile phone conversations and data in the nearby vicinity.

Aftenposten said in its report that it did not know who had set up the spying posts, but that no Norwegian authority they contacted had claimed ownership of them – leaving open the possibility that they were set up by foreign agents for the clandestine monitoring of political goings-on in Oslo.

The director of Norway's National Security Authority confirmed after a preliminary investigation that Aftenposten's findings were probably accurate.

Then, Monday Dec. 15, the Police Security Service (PST) said in a statement: “PST has today decided to initiate an investigation to examine whether the information that has emerged lately surrounding fake base stations concerns illegal intelligence activities for the benefit of foreign states cf. Penal Code. § 91a.”

Base stations are radio transmitters that link mobile phones to the telephone network. An IMSI catcher can act as a fake base station between mobile phones and the real base station, and thus be used to eavesdrop.