 Controlled demolition of a building to code requires weeks of preparation, including laying large quantities of explosive and cutting through beams, which would have rendered the building highly dangerous and which would have to be done without attracting the attention of the thousands of people who worked in the building.Controlled demolition is traditionally done from the bottom of buildings rather than the top, although there are exceptions depending on structural design.There is little dispute that the collapse started high up at the point where the aircraft struck.