Reply #27: What was loud explosion many heard 15 minutes after Pentagon crash?

[philb]

Other witnesses said the plane crash was followed by an explosion about 15 minutes later that could be heard miles away - apparently the sound of a large portion of the Pentagon collapsing.

http://www.americanmemorials.com/memorial/tribute.
asp?idMemorial=1316&idContributor=7466



Other witnesses put the collapse more than 15 minutes later?
So what was the loud explosion many heard?

And what caused the Pentagon collapse? The section where the collapse occurred doesn't show much fire damage.