A number of different local/state and federal agencies on site have ready access to handheld bomb residue detectors. These detectors were surely used the very first night, and their use has likely continued during the search & rescue the excavation. If there were any any indications of HE residue, excavation would have been halted, at least temporarily, and the ATF would have taken over and assessed how/if to move forward with the search & recovery.
The explosive residue - if any - is at least 12 layers BELOW the rubble that is currently being cleared away.
The only way a concrete deck building can collapse straight down is if the primary support columns all fail at exactly the same moment.
Are there alternative explanations? I will certainly listen if there are.
At the moment, however, the only explanation that makes sense to me is a deliberate act of terrorism.