I am pretty sure there are ways to distinguish between plastique and various 
petrochems exploding.  They explode quite differently.  For example sewer gas 
probably would not destroy the sewer cap... instead it would knock it up into 
the air.  On the other hand c-4 would frag the sewer cap.

As for potassium and chloride... explain to me again what makes that special?  I 
mean, sodium is a cation of the same family (group?  I forgot which it is 
called) and chloride is well, chloride.  So why doesn't table salt kill us?  I 
am more knowledgeable about explosives than poisons.  And just because such and 
such ion exists in the body already doesn't mean it can't be detected as a 
poison!  The body has CN- (cyanide) ions, as well as trace amounts of gold and 
other random crud.  If someone found a half kilo of gold in someone's blood 
stream people would probably notice...

I have heard of several drugs that induce heart attacks, and while the CIA might 
know otherwise, all the ones I am familiar with are traceable.

My father is a pathologist, and used to do autopsies... out of curiousity I used 
to ask him what chemicals one could use to kill someone without it being 
traceable as an actual intentional poisoning rather than an accidental one.  He 
said it is pretty difficult, as techniques are sophisticated these days.

Ben Williamson