The Bam (Iran) earthquake of 2003 resulted in one of the worst human disaster in recent years. Yet the magnitude of the event - Mw = 6.6 - was relatively moderate. We show that the remarkable recording of the ground motion produced in the city itself contains some clues which help explain this disaster. We identify three factors whose unfortunate combination led to the strong ground shaking which destroyed the city: 1) The Rayleigh-like speed of the rupture, 2) The high slip velocity, which exceeded 2m/s over a large part of the fault, 3) The strong directivity, which focused the elastic energy released directly toward the city.