The story...

Testimony of WTC survivors shows there were secondary explosions on 9/11.

Our take...

As we say elsewhere, not every account of an "explosion" or "bomb" means exactly what you'd think. Take the case of Kim White, for instance, whose story still appears in many places as indicating "bombs in the building":

"Kim White, 32, an employee on the 80th floor, also reported hearing an explosion. "All of a sudden the building shook, then it started to sway. We didn't know what was going on," she told People. "We got all our people on the floor into the stairwell . . . at that time we all thought it was a fire . . .We got down as far as the 74th floor . . . then there was another explosion."
www.thetruthseeker.co.uk/article.asp?ID=245

There's an immediate problem with this account. White talks of the second explosion occurring when she was on the 74th floor, yet she still managed to leave the building. Controlled demolition doesn't normally have a 30 minute plus delay before bringing down a structure, so is there another possible explanation?

As it happens, there is.

"... The door to the second bank of elevators slid open and I stepped out onto the 80th floor. As other co-workers came into this side of the office (which we had dubbed the West Wing) we said hello to one another... Kim White Sales Administrator..."
http://www.webscope.com/~larrygc/gra/wtc/sep11wtcdisaster.htm

This is the story of Susan A.F., a co-worker of Whites, and so well placed to look for an alternative take on events. Here's what she says on the above page:

"We were ushered into the Port Authority office on the 78th floor... As I turned to watch some of my fellow co-workers making phone calls, there was a second ka-boom, the building shook again and debris started hitting the windows... I thought some part of the plane or some part of the building that had been hit by the plane had exploded and debris was sliding down from the floors above us. I would later learn it was a second airplane diving into the other tower and it was debris from that explosion hitting the windows".

So for her the explosion is consistent with the second impact. Doesn't prove it was, but this interpretation gains support from a couple of other accounts.

As soon as we arrived, 84, a massive explosion goes off, and at this point we didn't know what it was. We thought it was a secondary explosion. We didn't know that it was a second plane. In fact, I didn't know there was a second plane until much later in the evening.
http://www.npr.org/documents/2005/aug/fdnyrecords/delgado.pdf

We figured by the time we got to the fifth or sixth floor, that’s when the south tower was hit. I had no idea the south tower was hit, and I don’t think that Chief Jonas – Captain Jonas at the time – or Lieutenant Foti knew at that point either. I remember the whole north tower literally vibrated. The only way I can explain it is if you were at the edge of a subway platform and the train was coming in, you felt that wind and the sound, but with an added effect like the floor vibrated. Everybody just cringed and really was not sure what was going on. I just assumed that it was something above us. I had no idea that the south tower was hit.
http://www.firehouse.com/terrorist/911/magazine/gz/blaich.html

"As we got near the top of the escalator that brings you to ground level from five floors below, we heard what sounded like a bomb going off," Seebode said. "It was the second plane hitting World Trade Center."
http://www.hq.usace.army.mil/cepa/pubs/nov01/story18.htm

This first two accounts are particularly interesting, as they suggests that many people would have believed, for some time, that there was a secondary explosion. Are any other "bomb in the building" quotes possibly referring to the impact of the second plane? We don't know (and there's no question that the vast majority don't refer to this), but the story illustrates another point relating to these testimonies. It's important to know when, and where the possible bomb was heard: short clips like "it sounded like a bomb went off just as I left the towers" aren't good enough.