The story...

There's something very strange about the way the Pentagon lawn was speedily covered with sand and gravel after the attack. What were they trying to cover up?

Our take...

The November 2001 edition of Civil Engineering Magazine explains all:

Ed Pickens, the senior construction scheduler: "On that first day we discussed with the FBI where to place the dumpsters needed to cart away debris. We discovered, however, that the dumpsters had to be brand new because the debris was evidence and could not be contaminated in any way. So we had to deliver numerous new dumpsters to the site immediately. And then I informed the FBI that we were going to have to build a road for the trucks carrying the debris because the ground around the heliport—the area closest to the blast—was too soft.

The FBI authorized construction of the road, and I called a contractor, who got the gravel, and we got things moving"
http://www.pubs.asce.org/ceonline/ceonline01/0111feat.html