Here are some beams from the WTC rubble. There was no reason for cleanup crews to do this kind of cut unless they wanted the beam to come down on their heads.
That kind of cut is made with a linear shaped charge, and it is used in demolitions, it makes the top part slide over the bottom part and the building falls :
Mystery solved.
The only way to find out the truth is to find some clean up people and ask them about those beams