I'd have to say explosive/incendiary-devices. There was opportunity for them to be placed during a two-week period prior to the event. They are the only logical explanation under the failure of basic physics to definitively satisfy the questions.
What leaves me unsettled now are the follow-up questions. WHO is responsible? Why?
That's where the tin-foil hats come on though, and I have no idea how to wade through such a cesspool of misinformation, hatred and ignorance. Even so, one can find many plausible hypotheses from the socio-economic implications in the aftermath, but it is beyond my ken to definitively identify a sum motive.
There are too many branches and not enough light to get a clear picture of the roots.
Since we don't have video evidence from inside and the buildings weren't wired with sensors, the questions are impossible to definitively answer. All we can do is theorize. Our theories may not be entirely accurate. Perhaps the planes caused more core damage than we're estimating; perhaps there was more combustible material near the core than we're estimating; perhaps the effects on the fireproofing were worse than we're estimating. But I think the timeline works against the idea of explosives:
WTC 1: North (first hit)
10:06: NYPD officer warns that the building is going to come down and advises pulling emergency vehicles back from the building.
10:20: NYPD helicopter reports that the top of the tower is leaning.
10:21: NYPD helicopter reports that tower is buckling on southwest corner and leaning to the south.
10:27: NYPD helicopter warns that the roof is going to come down very shortly.
10:28: Building collapses.
Comment: The helicopter reports the top is leaning 8 minutes before the building collapses; it reports buckling and more leaning; it warns that the top is going to come down; and then it does. Wouldn't a detonation of explosives just bring the thing down? Instead we have an 8 minute span.
WTC 2: South (second hit)
9:37: 911 call says a floor collapsed below people in the 90s; they moved to 105.
9:41: NYPD dispatcher tells units someone on 106 reports that the floor is collapsing.
9:51: NYPD dispatcher tells units that people on 106 report that the floor is crumbling.
9:52: NYPD helicopter warns large pieces are falling from the top of the building.
9:58: NYPD helicopter warns that the building is going to come down.
9:59: Building collapses.
Comment: The 911 calls are less reliable than the helicopter, I'd say, because people are terrified they're about to die. If accurate, there's some kind of floor collapse in the 90s at 9:37; then there's another event on 106 at 9:41; then there's evidently a more serious event at 9:51. Here, the helicopter can verify, noting that large pieces are falling off the top (right after the report from 106 that the floor is "crumbling"). The helicopter warns it's going to come down, and then it does. We have a 28 minute span from the first report that a floor had collapsed.
In both cases, witnesses throughout the building are calling 911 and family members reporting fires and heavy, worsening smoke that's forcing people to break windows; they also report that the sprinklers aren't working; and some of these calls were terminated on the collapse, with no audible explosions and no reports of explosions from the people who were on the calls. In the call posted in this thread, for example, the caller just screams out before the call is terminated, presumably because a floor is falling down on top of him or because a floor gave out beneath him.