Murder on the Thirty-First Floor — Per Wahloo

In an unnamed country, in an unnamed year sometime in the future, Chief Inspector Jensen of the Sixteenth Division is called in after the publishers controlling the entire country's newspapers and magazines receive a threat to blow up their building, in retaliation for a murder they are accused of committing. The building is evacuated, but the bomb fails to explode and Jensen is given seven days in which to track down the letter writer.
 
Jensen has never had a case he could not solve before, but as his investigation into the identity of the letter writer begins it soon becomes clear that the directors of the publishers have their own secrets, not least the identity of the 'Special Department' on the thirty first floor; the only department not permitted to be evacuated after the bomb threat.