NYT reports that it was a black powder device that used kitchen type egg timers to set them off:http://www.nytimes.com/2013/04/17/us...it_th_20130417The pressure cookers were filled with nails, ball bearings and black powder, and the devices were triggered by “kitchen-type” egg timers, one official said.
People keep saying these were 'rudimentary' devices so as to denigrate them. It seems to me that making a device out of commonly available materials is a good way to avoid leaving a trail or drawing attention while preparing. The materials in this may have been rudimentary but the effects and the malevolent brain work were not.
The materials used in 9-11, box cutters, were rudimentary but the concept behind the op and the effects were not.
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