I believe, based on the Naudet brothers' documentary that shows the lobby all blown out, the eyewitness accounts of William Rodriguez and many others, the seismic activity, the way the North Tower's antenna fell first, and many other pieces of evidence, that the core was destroyed first, specifically the basement floors where the tower meets bedrock. Later, during the collapse, there are many places where fragments of what may be glass are breaking out ahead of the collapse zone. Additionally, in at least one (CameraPlanet) video, there is what appears to be molten metal dripping out of one corner of the building, right at the support structure. It is also supported by eyewitness accounts from a reporter. I'm not familiar with the available methods of detonation. Anything I could say to that would be speculation.