There were no bombs involved. They are mistaken. Case in point.
The Elevator Man's Tale
What we heard was 6 and 7 car free-falling from the 107th floor and they impacted the basement at B-2 Level.
And that’s the explosion that filled the lobby within a matter of two or three seconds, engulfed the lobby in dust, smoke.
Jay Swithers
An ambulance pulled up which was very clean, S0 I assumed that the vehicle had not been in the what I thought was an explosion at the time,
but was the first collapse.
Dominick Derubbio
t was weird how it started to come down. It looked like it was a timed explosion, but I guess it was just the
floors starting to pancake one on top of the other.
Craig Carlsen
Craig Carlsen said that he and other firefighters “heard explosions coming from . . . the south tower
...there were about ten explosions. At the time I didn't realize what it was. We realized later after talking and finding out that it was
the floors collapsing to where the plane had hit.
In other words, the sounds they heard had nothing to do with explosives. I might add that manhole explosions are common in New York City and that non-bomb related explosions have occurred at the WTC even before 9/11 because New York City experiences over 2000 explosions each year. Other explosions were attributed to exploding gas lines as confirmed by FDNY firefighters during an NBC news playback two years ago on TV.
A couple of examples in the following reports.
Electrical Fire Hurts 6 at Trade Center
July 24, 1992
An air-conditioning transformer five stories below the World Trade Center caught fire after an explosion last night, the authorities said.
The Day The World Shook
February 26, 1993
"...Engine Company 10, whose quarters are across the street from the World Trade Center, called us via radio and reported a possible transformer vault explosion on West Street near the Trade Center.
Transformer vault (also called manhole) explosions are fairly common place in Manhattan, especially during wet weather. They're highly visible and normally generate numerous telephone calls to the Central Office.
http://www.fdnewyork.com/wtc.asp