JACKSON, Miss. — Six of seven Mississippi chicken processing plants raided Wednesday were “willfully and unlawfully” employing people who lacked authorization to work in the United States, including workers wearing electronic monitoring bracelets at work for previous immigration violations, according to unsealed court documents.

Federal investigators behind the biggest immigration raid in a decade relied on confidential informants inside the plants in addition to data from the monitoring bracelets to help make their case, according to the documents.

The sworn statements supported the search warrants that led a judge to authorize Wednesday’s raids, and aren’t official charges, but give the first detailed look at the evidence involved in what Immigration and Customs Enforcement officials have described as a yearlong investigation.

Officials arrested 680 people during Wednesday’s operation targeting seven chicken processing plants in Mississippi.

The statements allege that managers at two processing plants owned by the same Chinese man appeared to be actively participating in fraud. They also show that supervisors at other plants at least turned a blind eye to evidence strongly suggesting job applicants were using fraudulent documents and stolen or made-up Social Security numbers.

The documents say federal officials have evidence from electronic monitoring bracelets that people who already had been arrested for immigration violations and weren’t allowed to work in the United States were working at all seven plants that were raided.

The number of criminal convictions for hiring people without documents has historically been low because prosecutors must prove employers knowingly hired someone in the United States illegally. Employers often say they were fooled by fraudulent documents.

Companies can also face administrative fines based on audits of I-9 forms, which American employees fill out when they’re hired, presenting documents meant to prove they can legally work in the country.

Investigators allege the most brazen fraud took place at two smaller chicken processing plants — PH Food Inc. in Morton and A&B Inc. in Pelahatchie. Sworn statements identify Huo You Liang of California, known to his Mississippi employees as Victor, as the owner of both.

A PH Food employee, acting as a confidential informant, told Homeland Security investigators that the vast majority of the 240 employees at PH’s plant in Morton and the 80 employees at A&B’s plant in Pelahatchie didn’t have proper work documents.

Calls to A&B and PH Food on Friday went unanswered.

Jeff Amy is an Associated Press writer.