Let's examine the logistical requirements of planting 20,000lbs of thermite or conventional explosive. We must first determine that the physical transportation of materials onto the site is possible and that method is portable.

We'll begin by converting our units to metric for easier reading:

20,000 lbs
= 9090.9 kg
= 9.0909 Tonnes

9 tonnes may seem a lot, but as I'll soon show you, many hands do indeed make light work. Assuming 50 people were involved, that would require each to plant:

181.818 Kg

That would require 3 trips, carrying approximately 60kg per trip. Assuming a generic device size of around 3kg, that would be 20 devices per trip, or 60 devices per man in total. For a mental reference, the average weight for a male adult in the US is around 76-83 kg according to Wikipedia.

60 Devices * 50 people = 3000 devices

As each building had 110 floors, that gives us 220 floors in total. We can spread these 50 people so that we have 1 person for every 4 floors, across both buildings and ignore the bottom 10 floors, leaving us with 200 floors to cover.

Each man would then place 15 devices on each of the 4 floors assigned to him.

50 peopele * 4 floors = 200 floors
(15 devices x 4 floors) * 50 people = 3000 locations/devices

So, that's 1500 devices and 25 men per building.

Thus, it is feasible for there to be as little as 33 people involved if each carried 90kg. Gaining access has numerous plausible scenarios and only requires a window of 5 minutes. Given a good hiding spot for the devices, 1 person could move across 4 floors, positioning 60 devices, in 3-4 hours.

So, here is a quick table:

50 people - about 3-4 hours
100 people - about 1.5-2 hours
150 people - about an hour

Numerous other variations of this basic analysis will also function just as well. As you can see, it would be pretty easy for any organisation of sufficent size involved in organised crime.