‘You don’t get out much, do you?
Here’s a hint - the same cops that came out to help pick up the bodies of the dead children, are the ones who did the residue testing.
Do the math.’
Was the gratuitous insult necessary?
At the crime scene, a SCI takes samples, bags them, and delivers them to the crime lab. The testing is done there.
Funny, I felt your studied naivete' was the gratuitous insult. But whatever, if you really can't figure out what happened then let it remain a mystery. Better that than sully your delicate world.