At this point, nobody knows for sure where the aminopterin came from and whether it is the only cause of the poisonings.
ABC News reports that the aminopterin might have come from Chinese wheat:
Investigators, meanwhile, are looking into whether the rat poison came into the United States on an ingredient used in the recalled food. ABC News has learned that Menu Foods bought wheat gluten, the only ingredient changed in its plants, from China.
However, the AP (via CNN) quotes two experts who cast doubt on wheat as the source of the aminopterin.
The first, an expert in pest management, doubts that rodenticide could end up in wheat:
Bob Rosenberg, senior vice president of government affairs for the National Pest Management Association, said it would be unusual for the wheat to be tainted.
"It would make no sense to spray a crop itself with rodenticide," Rosenberg said, adding that grain shippers typically put bait stations around the perimeter of their storage facilities.
Andre Rosowsky, an expert in aminopterin, tells the AP that the food may have been deliberately tampered with:
Rosowsky speculated that the substance would not show up in pet food "unless somebody put it there."
According to the AP, Menu Foods says their products could not have been tampered with because the poison showed up in foods made at two separate plants. I think that really depends on your definition of "tamper." Perhaps poison ended up in the wheat because someone tampered with the wheat back in China or at some point before it reached the food plants.
Before we start looking for a culprit, another problem remains. We don't really know whether aminopterin is the cause or even a cause of the poisonings. ABC News quotes some skeptical vets:
"Renal failure is not the expected response to these drugs," said Susan Weinstein, executive director of the Massachusetts Veterinary Medical Association. She added that most rodent poisons work as severe anticoagulants — meaning they cause the rats that ingest them to bleed to death.
"Whether this particular toxin in this case can create renal failure depends on how this drug works in the body, which may be an entirely different pathway than the anticoagulants," Weinstein said. "Because we aren't yet familiar with this toxin, we can't be confident of the causation link."
Honestly, I hope this is the cause, because otherwise we're back to square one and the poisonings will continue.
According to both the AP and ABC News, Menu Foods continues to manufacture food in both of the affected plants. I guess they couldn't lose a day's profit or a night's sleep over this. For Menu Foods, the grinders of the giant pet food machine have to keep spinning, no matter what the cost in lives.