911 Research has high-resolution images of the North Tower, which display an interesting sequence of an emerging hooked beam out of the Tower in its destruction. For this beam to come out of the North Tower shows clearly explosives were in the Tower & used for its destruction.
Fig. 1 shows a hooked beam descending down, below the dust cloud. In Fig. 2 the front part of it swings toward the Tower, as it is still attached to the Tower. In Fig. 3 swings more towards the Tower, as looser junk behind it comes from the Tower. In Fig. 4 it separates from the junk behind it, & further tumbles over. Fig. 5 shows a completely tumbled over beam nearly 40 ft. long that it is 3D in nature, with its hook now on the left, & a regularity of the inhomogeneities in it.
The fact the beam survived emerging from the Tower implies it is probably metallic. The hook in the beam apparently came when it rammed into & broke through the Tower wall. The regular inhomogeneities were from its structure inside the Tower.
 "North Tower Exploding," from photos by Noah K. Murray, online at http://911research.wtc7.net/wtc/evidence/photos/wtc1exp21.html
 North Tower photo in C. Grabbe, How the WTC Was REALLY Demolished on 9/11 (Fig. 34) & National Swindle of the WTC (Fig. 5 of Chapter 3).