It was most likely an ALCM-class cruise missile, which is launched from aircraft. There is a photograph of such a cruise missile painted to look similar to a commercial passenger jet, albeit smaller. There is also the critical detail of an engine part found in the wreckage inside the Pentagon, an engine which could very likely been from said cruise missile.

There are witnesses to a "large plane" heading towards the Pentagon, then altering course and heading away after flying roughly overhead. Most everyone's attention was drawn to the explosion at the Pentagon, and few noticed this plane leaving the airspace.

I listened to a most interesting interview many years ago, of the taxi driver who had the street light fall onto his taxi. He changed his story twice, but was very nervous, and seemed to indicate, initially, that there were people who would harm him if he revealed certain details.