The World Trade Centre structural support consisted of 14 inch (360mm) wide steel columns, spaced 3 feet (~1m) apart along each external face of the building; 59 on each of the 208ft (63m) long external walls. 232 columns in total around the perimeter of the building. All of the building’s dynamic wind loads were taken by these columns.
In addition, there were was a central box core 135ft x 85ft (41m x 26m) containing the building services and an additional 47 steel columns. It was these columns that supported the bulk of the building’s vertical loads. Even if enough supports had been removed to make the building collapse, the direction of that collapse would still have been vertically downward through the core of the building; in other words, the building was never going to topple over regardless of how big the terrorist bomb was. We saw that with the September 11 attack; when the columns yielded, the building collapsed vertically inwards on itself.
The columns were cross braced, forming four boxes, each box incorporating one external corner and part of the core, with considerable overlap; removing the supports under one of these boxes would have resulted in the loads being distributed across the remaining supporting columns.
There’s another point working against the terrorists: steel columns are
not particularly vulnerable to blast. Here’s a photo of the result of the 1993 bombing:
What you can see is that, although the support points where the reinforced concrete floors connected to the columns gave way, the columns themselves are virtually untouched.
When the buildings were hit in 2001, even with massive amounts of structural damage and large numbers of columns destroyed, the building continued to stand for nearly an hour until enough of the remaining columns began to yield in the heat, at which point the building collapsed.
So summing up:
no, they were not going to bring down a tower. Not without multiple bombs, strategically spaced around the building’s basement at critical structural points, and even then the building would have collapsed
vertically, not topple over.