There are several informative items in Wikepedia:
1) On the collapse
http://en.wikipedia.org/wiki/World_Trad ... y_theories
http://en.wikipedia.org/wiki/Collapse_o ... ade_Center
They discuss the following in particular in regard to 7 WTC
Following a three year investigation NIST released its final report on the collapse on November 20, 2008.[59] Investigators used videos, photographs and building design documents to come to their conclusions. The investigation could not include physical evidence as the materials from the building lacked characteristics allowing them to be positively identified and were therefore disposed of prior to the initiation of the investigation.[56][60] The report concluded that the building's collapse was due to the effects of the fires which burned for almost seven hours. The fatal blow to the building came when the 13th floor collapsed, weakening a critical steel support column that led to catastrophic failure, and extreme heat caused some steel beams to lose strength, causing further failures throughout the buildings until the entire structure succumbed. Also cited as a factor was the collapse of the nearby towers, which broke the city water main, leaving the sprinkler system in the bottom half of the building without water.
NIST considered the possibility that 7 WTC was brought down with explosives. It concluded that a blast event did not occur and that the "use of thermite [...] to sever columns in 7 WTC on 9/11/01 was unlikely".[56] The investigation noted that no blast was audible on recordings of the collapse and that no blast was reported by witnesses, even though it would have been audible at a level of at least 130-140 decibels at a distance of half a mile. NIST also concluded that it is unlikely that the quantities of thermite needed could have been carried into the building undetected. The theory that fires from the large amount of diesel fuel stored in the building caused the collapse was also investigated and ruled out
2) on the conspiracy theories in General
http://en.wikipedia.org/wiki/9/11_conspiracy_theories
Yes there was a conspiracy: it involved Arabic terrorists, associated with an organisation that had tried and failed before to topple the buildings: the suggestion that Arabs/bedouins were incapable or organising it is a somewhat racists remark, the fallacy of which is revealed by the fact they succeeded.