Debris and wreckage maybe? Tests for explosive residue are normally conducted on material that has been, shall we say, traumatized.
I understand that...I wasn't trying to be wise-guy.
Nor am I buying into an internal explosion theory.
When they collapsed, it was said the concrete was pulverized into powder.
Where would they even start to test ? with which debris, where?
and why would they even look/test for internal explosives ?
if the jet fuel supposedly melted the steel to weaken the structure etc...wouldn't it also have melted the explosive residue ?