Steve people have to remember that this wasn't the first attempt the WTC. Without checking the facts I think it was 1993 when car bombs were detonated underneath one of them causing quite a bit of damage. The WTC was a symbolic of the US, along with Pentagon and White House. All were targeted on 9/11 from information available. I'm sure it was never the intent to crash the 4th plane into a field. But hey it could have been A3's with great whopping big wheels!
