Immigration officials have raided multiple chicken plants in Mississippi, detaining 680 workers in what officials are calling the largest single-state sting in American history.

As part of an ongoing criminal investigation, 650 agents from Immigration and Customs Enforcement (ICE) executed federal search warrants at seven processing facilities outside Jackson, Mississippi, seizing business records and rounding up hundreds of workers who the agency said were unlawfully working at the plants. 

The scene was wrenching. In Morton, Mississippi, detained workers, with their hands zip-tied behind their backs, were bussed from a Koch Foods plant to a nearby military hangar to be processed for immigration violations. The Associated Press reports that people congregated outside, waving goodbye and shouting “let them go.” As school got out, children of the arrested were put up in a local gym, where they were picked up by neighbors and strangers.

Matthew Albence, the acting director of ICE, said the raids were part of a year-long investigation. Mississippi state law requires employers to check the immigration status of workers through E-Verify, a federal system that matches an I-9 against social security numbers.

“To those who use illegal aliens for a competitive advantage or to make a quick buck, we have something to say to you. If we find you have violated federal criminal law, we are coming for you,” said Mike Hurst, the U.S. attorney for the Southern District of Mississippi, at a press conference. 

The hardest, most dangerous aspects of chicken processing work, such as the cutting, cleaning, and packing in factories, is often done by undocumented immigrants. While unauthorized workers make up 5 percent of the United States’ overall labor force, they comprise 17 percent of animal slaughtering and processing workers, according to Pew Research. 

It’s those workers, legal advocates say, who usually bear the punishment of raids, and not employers who knowingly hire them. This was the biggest enforcement action since 2006, when immigration agents raided Swift plants in six states, sweeping up more than 1,200 people. Two years later, in 2008, agents descended on the Agriprocessors kosher plant in Postville, Iowa, arresting nearly 400 workers.

Meanwhile, criminal prosecution of employers is rare, although a Tennessee slaughterhouse owner was recently sentenced to prison for ducking payroll taxes. No enforcement action against the Jackson-area processors has been announced.

Meatpacking is a tough industry. Labor advocates have suggested that companies struggle to find American workers who are willing to tolerate the sometimes deadly work, despite improving workplace injury rates. For those reasons, a report from Human Rights Watch found that meatpackers may deliberately recruit immigrants, who might not complain for fear of being deported.

“The industry knows that a workforce that is less protected would be less willing to complain about workplace abuses, and lower wages, and worse conditions than U.S. workers,” says Caitlin Berberich, an attorney at Southern Migrant Legal Services, in Nashville, Tennessee.

Some advocates believe some of the raids may be a form of retaliation; the Koch-owned plant has a history of labor unrest. In 2008, workers from the plant sued the Illinois-based company, alleging that managers were sexually and physically assaulting workers; charging them money to use the bathroom and leave work at the end of the day; and threatening them with deportation. The case was consolidated with a second lawsuit brought by the Equal Employment Opportunity Commission in 2011.

Last year, Koch settled with the workers for $3.75 million, and agreed to enter into a three-year consent decree, which required a 24-hour hotline for reporting discrimination complaints, among other things. The case is considered significant not only for the workers’ complaints, but for establishing a legal precedent that maintained the confidentiality of applications for U-visas, which are granted to crime victims.

Eunice Cho, an ACLU attorney who consulted on the litigation, says the plant’s high profile could have made it a target of the Trump administration, which has pledged to crack down on unauthorized workers. “There were threats made to call federal immigration authorities,” Cho says. “The irony today is we’ve actually seen that happened—that threat materialized.”

Koch Foods is one of the largest poultry producers in the country. It is the 135th largest private company in the United States, with over $3 billion in annual revenue, according to Forbes. The company, which has no relation to Koch Industries, did not respond to a request for comment.

Bryan Cox, an ICE spokesman, told The New Food Economy that the agency did not coordinate with Koch on the raid, adding that subjects of search warrants are not informed prior to their execution.