Dr. Stephen Sundlof, FDA chief veterinarian, told reporters melamine had been found in Chinese shipments of corn, wheat and rice gluten.
"That melamine was found in all three of those, it would certainly lend credibility to the theory that this was intentional," Sundlof said.
The chemical is used in the manufacture of plastics and fertilizers that elevates protein level readings, USA Today reported.
Pet food containing melamine has sickened and killed scores of cats and dogs in North America.
Sundlof said the agency has not been able to get letters of invitation from the Chinese government that would allow FDA inspectors to enter the country. The Chinese government has said the contaminated wheat gluten was not meant for pet foods and therefore was not its regulatory responsibility, The Los Angeles Times said.
The contamination has resulted in numerous North American recalls of millions of cans and pouches of pet food.