The 'powers that be' are grasping at straws.
First, they jumped on aminopterin - which was a non-issue at the levels they
found.
And, the same is true for melamine - it's essentially non-toxic at the low
parts-per-million levels that were
detected in some pet food samples.
LD50 for melamine in mice/rats is on the order of 1-2gm/kg body weight.
It's highly likely that no one will ever ascertain WHAT was in these recalled
pet foods that caused the small
numbers of pet deaths. There was a very similar incident several years back
in the far east, and the folks who
investigated that situation were never able to figure out what caused the
illness/deaths in affected cats/dogs.