WASHINGTON, May 11: US investigators found shuttered factories when they arrived at the Chinese food processors blamed for putting the chemical melamine into vegetable proteins shipped to America, officials said on Thursday.

The proteins were used in feed for pets, hogs, poultry and fish. All the suspect products have been traced to two makers in China, US Food and Drug Administration officials said during a news conference.

“We visited the two facilities and there is essentially nothing to be found because they are closed down,” said Walter Batts, head of FDA's office of international investigations. An FDA team has been in China since April 30.

“Nothing is available to be seen at the facilities. They were closed down, machinery dismantled,” said Batts.

Chinese officials did obtain samples at the facilities in April and sent them to an independent laboratory for testing, he said.

“We assume we will have access to those when the results are ready.”

FDA said it received “extensive cooperation” from China's General Administration of Quality Supervision, Inspection and Quarantine but also noted the agency had “limited authority” over the processors. Officials said they were satisfied the agency shared its information with FDA.

The FDA investigators are scheduled to return to the United States next week.

Separately, two senior Democrats in Congress asked US Trade Representative Susan Schwab in a letter whether the United States can challenge the food rules of other nations “based on evidence they are not meeting international standards and may be endangering public health in the United States.

“China is especially poor at meeting international food safety standards,” wrote Connecticut Rep. Rosa DeLauro and Assistant Majority Leader Richard Durbin of Illinois.

China shipped $2.3 billion in agricultural products to the United States in 2006.—Reuters