By Charles Abbott

WASHINGTON, May 10 (Reuters) - U.S. 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, U.S. 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 U.S. 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.

Skretting Co., based in Vancouver, Canada, announced a recall of fish feed that may contain the industrial chemical. The feed was shipped to at least seven fish hatcheries in the U.S. Northwest. A sample taken from a hatchery in Oregon tested positive for melamine.

Two commercial fish farms will be checked to see if they got fish feed containing melamine, said FDA. The agency was tracing how much of the feed was distributed and how widely.

"Humans who may have eaten fish containing the melamine-contaminated fish feed have a very low health risk," said Dr. David Acheson, assistant FDA commissioner for food protection.

Another FDA official, Stephen Sundlof, said melamine is "very water soluble" and is flushed from animals quickly. As a result, it was unlikely to accumulate in muscle meat, he said.

Scientists are conducting an exposure assessment as part of deciding what to do with hogs and chickens that are being held on a number of farms.

The FDA has restricted imports of all vegetable proteins from China and is not releasing the material until shippers provide documentation of tests showing the protein to be free of melamine, Acheson said.

The agency also is testing all imports of pet food from China for melamine contamination, and plans to begin sampling imports of fish feed from China soon, he said.

In the United States, the FDA is visiting manufacturers and processors who use vegetable protein concentrates to sample for melamine contamination. It also is doing some targeted sampling of domestically made human food, he said.

The FDA also confirmed that a second U.S. company, which it said was based in Illinois, was found to be distributing the same contaminated material as ChemNutra.

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