The official hypothesis for the collapse of WTC 7 is explained
by NIST in its interim
"The initiating cause or causes of this collapse, and its sequence of events, are still being investigated though fire appears to have played a key role and there may have been some physical damage on the south side of the building."
However, Architects and Engineers for 9/11 truth web site have determined that the building was in fact brought down by controlled demolition. They created 2 lists of facts to make their case which can be seen here.
Arguments against controlled demolition and their counters can be seen here.