Congress ordered the U.S. Agriculture Department nearly 20 years ago to investigate how often pork plants slaughter hogs that arrive too sick or exhausted to stand.

The goal of the mandate was to develop ways to humanely handle the animals — and to determine whether they should be kept out of the food supply. That never happened.

Now, as dozens of U.S. pork plants are poised to begin a new safety inspection system, animal welfare groups and some members of Congress are trying to stop lame and fatigued animals from being turned into food for humans. The new program makes plant workers — not USDA inspectors — responsible for evaluating the health of pigs as they arrive at the facilities.

On Thursday, Farm Sanctuary and six other animal protection groups filed a federal lawsuit that seeks to make it illegal for pork plants to slaughter the animals, typically referred to as “downers” or “downed” pigs because they are unable to stand or walk when they arrive at the plants.

Rep. Earl Blumenauer (D-Ore.) said he supports the groups’ efforts and is working with other lawmakers on a plan that will include a public education campaign and renewed efforts to pressure the USDA to take action.

“It’s shameful that this is where we are at. These are sick, damaged animals,” Blumenauer, a member of Congress’s Animal Protection Caucus, said in an interview. “It’s ludicrous that this is still an issue and that the industry is now in charge of [evaluating] the animals. We will ultimately win like we did with downer cows.”

The USDA made it illegal in 2007 to slaughter downed cattle for beef products and in 2016 outlawed the slaughter of downed calves for veal. Agriculture officials cited safety concerns because the lame animals are more likely to harbor and transmit food-borne diseases in beef. The USDA also said that allowing their slaughter “may have created an incentive for establishments to inhumanely attempt to force these animals to rise.”

The agency’s inspection records show plant workers in recent years have kicked, shocked and dragged downed pigs in an effort to get them to stand upright. Mobilizing pigs, even for a few minutes, means the animal can be rendered fit for slaughter and be turned into pork products shipped to grocery stores and restaurants.

The USDA declined an interview request and did not answer written questions, saying the agency could not comment because there is pending litigation on the matter.

The industry estimates that about 500,000 pigs unable to walk or stand arrive at pork plants each year. The animal welfare groups cite industry-sponsored research that places the number closer to 1 million. It’s unclear how many of those downed pigs are removed by inspectors and not slaughtered.

The number of downed pigs represents a tiny fraction of the 124 million market hogs that are slaughtered annually, but there is no way for consumers to know whether the pork they eat came from a downed pig.

When it comes to livestock, no other animal in the United States is slaughtered for food more often than pigs. Market hogs — 5 to 6 months old and uniform in size — represent about 75 percent of all slaughtered livestock, well behind cattle, which represents 39 percent, and goats and sheep, which round out the remainder, industry records show. About one-quarter of U.S. pork is exported. (The USDA does not consider chickens and turkeys to be livestock.)

The National Pork Producers Council contends that most of the downed pigs are not sick and simply need time to rest. They suffer from stress, often during the journey from farms to pork plants, which can cause some to develop a condition known as fatigued hog syndrome. A similar malady does not exist for cattle.

“It’s a metabolic state. Recovery typically takes place within two hours, but they will fully recover,” said Daniel Kovich, the council’s assistant director of science and technology. Kovich said he could not say what percentage of downed pigs have the syndrome but believes it is “the vast majority.”

Farm Sanctuary has previously attempted to get the USDA to adopt a slaughter ban for downed pigs. In 2014, it filed a petition with the agency, asking it to follow the course it had already taken with cattle.

Five months ago, the USDA denied the petition, saying existing federal regulations are sufficient to keep diseased pigs out of the food supply. They also said USDA inspectors are well positioned in plants to prevent immobilized pigs from being mistreated or tortured.

The next day, the USDA said it had finalized the new inspection system that gives plant workers the responsibility for evaluating the health of pigs as they arrive at the facilities.

The Centers for Disease Control and Prevention estimates that pork containing pathogens leads to about 525,000 infections, 2,900 hospitalizations and 82 deaths annually. The USDA’s Office of Inspector General has issued numerous reports over the past decade criticizing the agency for its failure to stop the inhumane treatment of pigs in pork plants.

Efforts to prevent the slaughter of downed livestock date to 1986, after Farm Sanctuary founder Gene Baur rescued a sheep at a Pennsylvania stockyard that had been left for dead.

Baur used the story of the downed sheep — which he nursed back to health and named Hilda — to persuade stockyards to euthanize downed livestock instead of selling them for slaughter. But the voluntary “no-downer policy” didn’t hold.

“Culturally, the agriculture industry is very resistant to hearing from animal protection organizations that are challenging their practices. There is a tendency to dig in,” said Baur, who has a master’s degree in agricultural economics from Cornell University.

In 1998, Bauer filed a petition with the USDA asking the agency to make it illegal for downed cattle to be slaughtered.

At the same time, members of Congress were alarmed by the rising number of downed livestock. In 2002, Congress amended the Humane Methods of Slaughter Act, ordering the USDA to investigate and submit a report to Congress that would determine the causes and scope of downed livestock. Congress said the USDA should consider proposing new rules to prevent downed animals from being treated inhumanely.

The USDA said it completed the report but would not release a copy to The Washington Post, saying that the documents are now “Congress’ records, therefore we are unable to share them.” Agricultural committees in Congress said they were unable to locate the report.

The agency did not take any action on Baur’s cattle petition until 2007 — after mad cow disease threatened herds. The disease affects the central nervous system of adult cattle, making it difficult for them to walk or stand. Keeping potentially infected cattle out of the food supply became imperative, because humans can develop a fatal brain disease if they eat beef from infected cattle.

Former congressman Gary L. Ackerman (D-N.Y.) remembers taking the fight to the House floor in the early 2000s, when he held up a picture of a crippled cow and said, “This does not a good steak make.”

Ackerman said he worries the USDA is about to repeat itself by not acting until a threatening disease strikes pigs. “We are not smart until a disaster happens, and then people become crazy concerned and take every precaution they should have taken in the first place,” he said.

Animal welfare and food safety groups share Ackerman’s concern, especially because some of the largest pork plants in the nation are expected to adopt the USDA’s new swine inspection system this year. Under the new system, plant workers will identify, isolate and sometimes remove diseased and downed hogs that arrive at the plants.

Kovich said the quality of these evaluations will remain the same.

However, animal welfare and food safety groups have expressed concern, because the USDA is not requiring any specific training for employees. The length and rigor of the training will be determined by plant owners.

The job of evaluating the health of pigs as they arrive at slaughterhouses has traditionally been conducted by USDA inspectors and veterinarians who are trained to spot signs of disease that could cause food-borne illnesses. They are also skilled at spotting contagious diseases such as African swine fever, which has recently decimated the pig population in Asia and has the USDA planning its own contingency plans should the disease strike in the United States.

Pat Basu, former USDA chief veterinarian, said the change is risky.

“This is like Russian roulette,” Basu said. “You might not get it, but you might. One disease condition can spread and ruin the entire country’s economy. It doesn’t matter if you eat pork or you don’t eat pork. It’s risky for everyone.”