WASHINGTON (AP) — The government acknowledged Friday that a shipment of peanuts from the plant linked to a salmonella outbreak contained a "filthy, putrid or decomposed substance" later identified as metal fragments. The shipment was returned to the U.S. in April, months earlier than reflected in a federal tracking database.

The rejected shipment — coming across a bridge between New York and Canada — was logged by the Food and Drug Administration but never tested by federal inspectors, according to government records. The incident took place in mid-September, the records show, weeks before the earliest signs of the outbreak.

The FDA said Friday that the shipment of chopped peanuts from Peanut Corp. of America in Blakely, Ga., was eventually destroyed, after back-and-forth efforts between the FDA and Peanut Corp. broke down and the FDA rejected as "unacceptable" findings by a private lab hired by Peanut Corp. to analyze its peanuts.

"The shipment was refused by FDA for filth," FDA spokeswoman Stephanie Kwisnek wrote in an e-mail to The Associated Press. "The importer requested to destroy the product." Another FDA spokesman, George Strait, said later Friday that metal fragments were found in the shipment.

"The FDA did everything appropriately in handling the activities associated with this shipment," Kwisnek said.

The FDA's explanation Friday raises new questions about the adequacy of food-safety tests arranged by Peanut Corp. of its own products. The FDA said it refused to accept the private lab analysis because of problems with the size of the sample tested, lack of information about whether experienced and trained workers conducted the test, and questions about whether the test could have detected certain types of metals.

The FDA, citing internal company documents, said Peanut Corp. had hired a lab that conducted at least 12 positive tests for salmonella between 2007 and 2008 at its Georgia processing plant, which has been identified as the source of the outbreak. The FDA said the company then used a different lab to retest the products, and those tests came back negative.

The chopped peanuts in the export case were prevented by the FDA from being allowed back into the United States because the peanuts contained an unspecified "filthy, putrid or decomposed substance, or is otherwise unfit for food," according to the FDA's report of the incident.

Peanut Corp. didn't immediately respond to AP's request for comment. Federal inspectors previously reported they found roaches, mold, a leaking roof and other sanitation issues at the company's processing plant in Blakely.

Members of Congress noted that the timing of the discovery of the adulterated peanuts came before the first clear signs of the salmonella outbreak that has sickened more than 500 people in the United States and may have killed at least eight. The FDA has since ordered recalls of a long list of products containing nuts from Peanut Corp.

"The FDA failing to follow up after this incident, does that mean that products that are not good enough for a foreign country are still good enough for the USA?" asked Senate Agriculture Committee Chairman Tom Harkin, D-Iowa. "That's a double standard that has deadly consequences for our citizens."

Harkin plans hearings on the outbreak and has proposed an overhaul of the nation's food inspection network.

The House Energy and Commerce Committee, led by Rep. Henry Waxman, D-Calif., announced oversight hearings will begin Feb. 11.

The head of the House appropriations panel that oversees FDA funding, Rep. Rosa DeLauro, called the discovery of the bad shipment "a perfect example of the patchwork system."

"Why was it able to get exported in the first place?" asked DeLauro, D-Conn. "That also begs the question, how many contaminated products are getting through our borders every single day? If the FDA discovered that there was an issue with this product inspection, why didn't they follow up on it? Why didn't they take a closer look at this facility?"

DeLauro said she wants the Justice Department to investigate the salmonella outbreak and is pressing for major changes in food safety oversight.

The government recorded the peanuts' seizure in the FDA's Oasis system, designed to prevent shipments into the United States of unsafe foreign products. In this case, it caught peanuts coming back into the U.S. after they were rejected abroad. According to the government's database, the FDA did not analyze a sample of the adulterated peanuts. The records show conflicting information about whether the FDA has a record of an analysis of the peanuts from a private lab.

The seizure of the peanuts is significant because it came before the salmonella outbreak, said Caroline Smith DeWaal, director of the food safety program at the Center for Science in the Public Interest in Washington.

"It strikes me that if FDA was paying attention to this information, that they might have gone and done an inspection of the plant in September instead of waiting until after the products were associated with a major outbreak," she said. DeWaal said she thinks "the question for the agency is how did they use it when it happened."

The incident was among nearly 1,400 around the country in September in which the FDA refused to allow shipments into or back into the United States, often because products are not approved for sale in the U.S. or were improperly labeled. In a few cases in September, the FDA detected salmonella on items coming into the U.S.

The rejected peanut shipment was stopped at a border crossing, apparently in Alexandria Bay, N.Y., suggesting the chopped peanuts had been sent originally to Canada. Canadian government officials told the AP they could not confirm the shipment.

Canada this week recalled several products as a result of the outbreak. The country is working with the FDA to trace back possible distribution of the products, said Garfield Balsom, spokesman for the Canadian Food Inspection Agency Office of Food Safety and Recall.

Associated Press writer Ricardo Alonso-Zaldivar contributed to this report.