The USDA announced this week that it was shutting down operations at a Chino-based meat producer, after hidden camera video showed workers there using various inhumane methods to force "downer" -- or non-ambulatory -- cattle to their feet and into the slaughter box.

Now, in the wake of the video's release and the agency's response, food industry insiders are questioning just how reliable the USDA's inspection process is. The incidents recorded at Hallmark Meat Packing occurred under the noses of eight on-site USDA inspectors.

"We rely on a system, and the system dropped the ball," said Dean Cliver, a food safety expert who has served in advisory roles with the Food and Drug Administration and the Department of Agriculture. "Somebody ought to be asking some questions."

USDA's Food Safety and Inspection Service indefinitely suspended inspection at Hallmark Meat on Monday, an action that effectively bars the supplier from slaughtering and producing meat. The agency ordered the suspension after ongoing investigations found the supplier's "humane handling" practices to be lacking, said Amanda Eamich, a spokeswoman for the inspection service.

Eamich said the USDA has yet to confirm that any downer cattle actually entered the food supply.

Cattle that are unable to walk are banned from use as human food because they show a higher occurrence of bovine spongiform encephalopathy, commonly known as mad cow disease.

Undercover activists with the Humane Society of the United States insist that downer cattle have entered the commercial food chain and that they have "very clear documentation" on video of at least four downer cows being slaughtered for human food.

One activist with the society, who worked at the Chino plant wearing a hidden camera, said federal inspectors were lax in conducting the screening for non-ambulatory cattle. The screening requires that cows walk from one pen to the next and back to prove that they are not sick or immobile. "It would take two or three of us to get the cow to stand in front of the inspector, on wobbly legs, and he would say 'That's fine,' " said the activist, who said such incidents happened about once a week during his six weeks at the plant.

The activist declined to give his name.

The activist said another pitfall in the system was the handling of cattle that collapsed after the pre-slaughter inspection.

According to the final ruling on downer cows issued last year by the inspection service, slaughterhouse employees are obligated to notify the inspector for a reevaluation if cattle become unable to stand or walk after inspection.

"When you read these rules and apply it to the practical workings of these plants, they're just absolutely not going to do that," the activist said.

Food safety experts said that even if downer cattle were introduced into the food supply, the risk of mad cow disease spreading was very low.

The real concern, they said, is the USDA's failure to detect and correct problems at Hallmark before the Humane Society released its video.

"If it's that apparent, as we saw on the tapes, the USDA inspector should have responded to that downer animal," said Michael Doyle, a professor of food microbiology and director of the Center for Food Safety at the University of Georgia.

Cliver, professor emeritus of food safety at UC Davis, said the suspension of the plant is "long past due."

"It's a shame when USDA has to read about this stuff in the newspaper before they take action," he said. Cliver said he was especially shocked by the news, because as someone who has worked on food safety for 45 years, he believed in the federal inspection process. "That the most intensive inspection system we have was asleep on this situation bothers me enormously," he said.

One retired food inspector, who once worked at Hallmark, said the USDA supervisor in charge of the plant had to have been aware of the practices shown in the Humane Society's video.

"The supervisor should have known what was going on," said Paul Carney, western council president for the National Joint Council of Food Inspection Locals, the USDA inspectors' union.

Bill Bullard, chief executive of the Ranchers-Cattlemen Action Legal Fund, an advocacy organization that represents cattle-raising farmers and ranchers, was also critical of the USDA's lax enforcement.

"We would hope that this example will impress upon the USDA the need to bolster its inspection processes to enforce the current law that prohibits downer animals in the human food supply," Bullard said.

Westland Meat Co., Hallmark's distributor and a ground beef supplier for the National School Lunch Program, has voluntarily halted operations, and school district officials around the country pulled suspect beef from lunch menus. Westland also supplied to several restaurant chains, including In-N-Out Burgers and Jack in the Box, which both severed ties with the supplier last week.

Richard Raymond, USDA undersecretary for food safety, expressed confidence in the department's inspection system.