Charges have been filed in federal court accusing more than 40 people detained in the recent Mississippi chicken processing plant raids of being in the United States illegally.

But more than a week after the raids, there are no records of company officials charged with knowingly hiring undocumented workers. This is despite information in federal search warrant affidavits suggesting company officials knew their workers were undocumented.

Some managers knew workers wore ankle monitors to work as they waited on immigration hearings, the warrants say. One of the chicken companies was aware its workers used fraudulent Social Security numbers, a confidential informant told investigators. A human resources employee revealed an employee was hired on two occasions, under two different identities.

When a Guatemalan man encountered law enforcement in Texas, he admitted he had worked at one of the plants, Koch Foods, and reportedly said the plant knew about his immigration status, and that there were “a lot of illegals working there."

Authorities said since 2002 they have recorded about 350 encounters or arrests of undocumented immigrants around the country who acknowledged working at either Koch Food or Peco Foods plants in Mississippi.

The initial wave of about 40 charges filed against workers this week includes an arrest warrant and a sworn affidavit by Homeland Security Investigations Special Agent Brent Druery. It accuses each person of either illegally reentering or failing to depart the U.S.

For example, an arrest warrant for Cesar Berduo-Perez said he had three prior encounters with immigration officials — including one in November 2011, near Morton, and another in 2013, in Scott County. When agents raided Pearl River Foods in Carthage last week, they once again found Perez, who is from Mexico.

Perez was deported to Mexico each time, the court documents say. ICE databases revealed Perez never received permission to apply for readmission. Federal law states someone illegally reentering the country can be fined or imprisoned up to two years, or both. They also can be deported.

The new criminal complaints filed against workers stem from the Aug. 7 raids of seven Mississippi food processing plants, when immigration officials arrested 680 people suspected of living and working in the country without permission. Officials called it the largest immigration operation of its kind in more than a decade, and the largest single-state raid ever.

Immigration fight:ICE raids in Mississippi: What we know now

The approximately 380 people still in custody were shipped to detention facilities in southwest Mississippi and Louisiana, authorities said.

Will employers face consequences?

Mike Hurst, U.S. attorney for the Southern District of Mississippi, has said the investigation is ongoing. He hasn't said specifically whether criminal charges will be brought against companies and officials — but pointed to a track record that includes several similar prosecutions.

The search warrant affidavits suggest all five companies that operated the processing plants knowingly hired undocumented immigrants. Companies engaging in a “pattern and practice” of knowingly hiring undocumented immigrants can be fined as much as $3,000 for each person hired. Company officials can face up to six months in prison.

Sometimes investigators leaned on confidential informants who talked with human resource managers and other company officials about what they knew.

In other cases, authorities gleaned information about company practices by questioning workers. For example, a Guatemalan employee working under an assumed identity said she was told during her job interview that it was OK to wear an ankle monitor, as long as it stayed charged. Another woman said she'd applied at one of the plants using an alternate identity and was "hired on the spot" for a three-month trial.

In addition, investigators found some employees had not been run through the legally-required federal E-Verify system, which scans an applicant's information through federal databases. (The system can't necessarily flag whether an applicant is using fake documents, however.)

Agents also said they learned some employees entering and leaving the plants did not match up with names of employees in the company's quarterly wage report filed with the state.

At PH Food in Morton, which employed about 240, an informant told agents nearly the whole staff was undocumented. The informant said the workers there used their real names, but fraudulent Social Security numbers. And the documents suggest PH officials knew, and leaned on a Louisiana payroll outsourcing firm to hide its activities. The same plant fired most of its remaining workers after last week's raids.

Lawyers and experts told the Associated Press that investigating management takes longer and is more challenging than detaining workers. "The employer says, 'I'm sorry, I didn't know they were unauthorized,'" explained Muzaffar Chishti, a senior fellow at the Migration Policy Institute.

In addition, experts say the documents, computers and other items seized during the raids likely will take time to sift through.

Tracking undocumented immigrants

The search warrants also show how authorities connected undocumented immigrants to the companies where they worked, and how they got hired in the first place.

In some cases, authorities tracked the immigrants via their ankle monitors, after they had been arrested by Border Patrol elsewhere around the country. Using GPS, agents watched as one woman remained at a Peco Foods facility in Bay Springs for as long as 10 hours per day. They tracked her as she traveled to work from her home in Laurel. And they did the same with many others.

At Pearl River Foods in Carthage, investigators served a subpoena for a list of employees. An analysis showed "numerous employees were utilizing stolen (IDs) to gain employment," including Social Security numbers that didn't match assigned names, and Social Security numbers belonging to dead people.

Investigators also staked out the facilities themselves. At Pearl River Foods, for example, they watched vehicles going in and out, and found only eight of 26 license plates belonged to people listed as current employees by the company.

In another case, authorities tailed a van leaving the Peco Foods plant in Sebastopol. They pulled it over after it reportedly crossed a center line, and found its six occupants had "unlawfully entered" the country.

Other times, investigators simply talked to employees or former employees. One Guatemalan woman admitted she’d entered the country illegally last year, that she’d been employed by Peco for about nine months, and had been hired after paying $600 for a fake Tennessee ID card and other documents.

Similar cases in Mississippi

Legal experts said there have been relatively few prosecutions of employers over hiring of undocumented workers over the years.

Under President Donald Trump, ICE has drastically increased immigration crackdowns including worksite enforcement actions, according to the Associated Press. But the amount of employers being charged has remained almost the same.

Ice raid:Chicken plant workers reportedly fired after MS ICE raid

There are a few prominent recent examples from Mississippi, however.

Howard Industries, one of the state's largest employers, pleaded guilty to knowingly violating the federal criminal conspiracy law in its employment of illegal workers at the company's electrical transformer plant in Laurel, following an investigation by ICE and Homeland Security. Nearly 600 suspected undocumented workers were detained in the 2008 raid.

Howard Industries waived indictment and agreed to pay a criminal fine in the amount of $2.5 million. The company's former human resources manager, Jose Humberto Gonzalez, also pleaded to federal charges tied to hiring of undocumented workers. He was given a sentence of six months of house arrest.

In another case, the U.S. Attorney's office filed a criminal complaint against the Country Club of Jackson citing immigration and Social Security violations. The complaint says the Country Club was knowingly employing illegal immigrants.

The case was settled, however, with the Country Club agreeing to pay a $214,500 fine, be audited by ICE and establish a training program to help employers detect fraudulent documents.

Contact Jimmie E. Gates at 601-961-7212 or jgates@gannett.com. Follow @jgatesnews on Twitter.