The WTC did not collapse because of the steel structures melting. When the first plane crashed into the building jet fuel leaked out from the fuel tanks. It then ignited and lit the area aflame. The fires reached temperatures of 1,832° F yet steel's melting point is 2,750° F. So the steel did not melt, rather it became more maleable. This was the cause of the building collapsing. As far as research has gone, there was no sign or evidence of an internal bombing and/or terrorist attack. However the jet fuel was not the complete cause of the fires. Once the fire was lit it spread. Items such as furniture, paper, and other tapestries caught flame and contributed to the fire.