On March 16, 1982, a U.S. marshal arrived at a North Carolina seafood processing plant, armed with a court order and intent on seizing 12 tons of scallops that the Food and Drug Administration had found to be infested with parasites.

He was, alas, too late.

More than two months had elapsed since the FDA had inspected the processing plant. By the time laboratory technicians had confirmed the presence of parasites a few weeks later, many of the scallops were swimming in cream sauce.

And by the time the marshal appeared with a seizure order in hand, there were no scallops left to seize.

According to a new General Accounting Office report, such incidents are far from rare as the FDA struggles to enforce a pure-food law that gives it no authority to keep contaminated products off the market while a formal seizure order works its way through the bureaucracy.

In 25 of 190 seizures reviewed by the GAO, "all or a large portion of the food had been sold before it could be seized," the report said. Most of the cases cited by the GAO involved food adulterated with "filth," meaning insect fragments and rodent or bird droppings.

The report, requested more than two years ago by a House Energy and Commerce subcommittee, is likely to spawn another attempt to tighten provisions of the 50-year-old federal Food, Drug and Cosmetic Act.

The FDA has been trying off and on for more than a decade to broaden its powers under the law, with little success. But in a letter to Henry A. Waxman (D-Calif.), chairman of Energy and Commerce's health subcommittee, full committee Chairman John D. Dingell (D-Mich.) suggested it was time to try again.

"The report documents glaring loopholes that make it impossible for FDA to ensure the public is not eating unsafe adulterated foods," he said.

Just last month, the GAO was congratulating Congress and the FDA for improving sanitary conditions in the food industry. A review of the FDA's records had found that only 7 percent of processing plants were failing sanitary inspections, compared with 40 percent in 1972.

But in the relatively few cases where filth is found, the new report suggests, the government isn't doing all it can to correct the problem.

According to the GAO, it takes the federal government an average of 65 days to process a seizure order, including three reviews at the FDA district level, three more at FDA headquarters and two at the Justice Department. Even a fast-track system that the FDA uses for some routine seizures, called "direct reference," takes an average of 47 days.

In the meantime, the FDA sometimes asks state officials to use their broader powers to halt food sales or works out voluntary agreements with food companies. That doesn't always work, the GAO said.

In 76 of the seizures in its study, food firms agreed to hold the product, the GAO said. But 19 of them reneged and "distributed or sold all or part of the food while awaiting FDA's seizure action."

In February 1982, for example, the FDA attempted to seize more than 6,000 jars of peanut butter contaminated with insect fragments. Although the company had agreed two months earlier to hold the product, all of the peanut butter had been sold. A later recall turned up only 237 jars.

Among other problems turned up by the report: The FDA does not always verify that products voluntarily recalled by processors are destroyed or reprocessed to make them safe, even when the contamination could cause serious illness or death.

One of the cases involved a massive recall of canned salmon in 1982, when the FDA discovered that defective canning equipment was puncturing small holes in the cans. The defect came to light when a Belgian man died from botulism after eating the salmon.

All of the recalled cans were to be visually inspected before being redistributed. But one of the nine Alaskan canneries involved shipped 140 cases without inspecting the cans, the GAO found, and another firm shipped the salmon overseas in the belief "that this requirement did not apply to salmon being exported." Because the FDA has no authority to review manufacturer's production or distribution records, food inspectors sometimes have resorted to gumshoe tactics in an effort to enforce the law.

In one case, an FDA investigator had to follow a delivery vehicle across a state line to document interstate shipment, the GAO said. In another, the FDA was forced to leave 228 cases of filth-contaminated cookies on the market because the manufacturer refused to tell inspectors where the cookies had been sent. Firms turning out contaminated food products because of unsanitary processing conditions are getting by with a light slap on the wrist, partly because the maximum fines allowed under the food and drug law have not been raised since 1938.

In one case, the GAO said, the FDA prosecuted a crab-packing company after 13 inspections in 11 years repeatedly found rats in the plant, flies on the crabmeat and employes retrieving crab claws from the floor. The owner was fined $3,500 and received a suspended one-year prison term, but three subsequent inspections found that conditions had not changed.

In another case, a commissary with $5 million in annual sales was fined a total of $1,300 after a series of inspections found rodent droppings in bags of rice and doughnut mix and a live mouse inside a bag of flour. The company had repeatedly promised to improve conditions, but never did.