In a NBC news video there was an interesting theory. It was suggested that the idea of crashing an airliner into a city was a very old idea
Possibly the closest this ever came to happening before September 11 was on April 7, 1994, when about-to-be-fired FedEx flight engineer Auburn Calloway, riding in the jump seat of a DC-10, attacked the three crewmen with a hammer and attempted to kamikaze the plane into FedEx's facility at Memphis International Airport. Calloway very nearly succeeded; the plan was foiled only because the other crewmen, all of whom were seriously injured by the assault, were able to muster up enough strength to fight him off.
Dave Hirschman's 1997 book Hijacked has a description of what might have been that sounds even more chilling in the aftermath of September 11:
A fully loaded DC-10 with twenty thousand gallons of fuel on board would be an awesome weapon. Calloway could plunge it into the FedEx package-sorting hub at Memphis International Airport and instantly turn the sprawling complex into an inferno ... By coming in fast and low, on about a 3-degree trajectory, he could knock out the entire FedEx facility ... Dozens of jet aircraft parked on the FedEx ramp would be reduced to aluminum shrapnel. The blast would ignite massive underground jet-fuel storage tanks, which probably would burn for days. Fed Ex executives would get the message that a catastrophe had happened when the explosion blew out windows in their three-story headquarters less than five miles away.
"Let me think about it" = the coward's way of saying "no"