(CNN) As federal indictments stack up against dozens of undocumented immigrants rounded up in workplace raids in Mississippi , no charges have been filed against employers who immigration officials believe knowingly hired them, according to online court documents.

Two of the raided chicken processing plants say they followed local and federal law and participated in E-Verify. Officials at two other plants did not respond to a request for comment.

Affidavits made public after the historic August 7 raids showed federal agents suspected the food processing plants unlawfully hired undocumented workers. But Mike Hurst, the US attorney for the Southern District of Mississippi, declined to comment on whether employers will be charged, citing the ongoing investigation.

"If you look at the history of this office, we have consistently prosecuted employers, companies and owners when evidence has been presented to us to prove beyond a reasonable doubt that they have violated federal criminal laws," Hurst said in a statement.

About 680 undocumented immigrant workers were detained earlier this month at plants across Mississippi in the largest single-state immigration enforcement operation in history. The detentions left friends, neighbors and, in some instances, strangers to temporarily care for the crying children of workers after the first day of school.

Two weeks later, nearly 40 workers have been indicted on charges ranging from illegal re-entry to misuse of social security numbers, according to court documents.

Still, no charges have been brought against companies or managers in what experts acknowledge can be time-consuming cases.

"It does take time to investigate the companies, but in these cases, the affidavits ... say the companies knowingly and willfully hired people who were undocumented or the documents they were producing were false," said Cindy Eldridge, a former assistant US attorney under Hurst who spent last week offering free legal services to detained migrants and their families.

Eldridge, who left the US attorney's office in February after two decades as a prosecutor, criticized the raids as "senseless" in a lengthy Facebook post this week.

"My sense is probably some employees will be prosecuted -- supervisory employees," she told CNN. "I'd be shocked if any of the owners are prosecuted."

Criminal prosecutions against employers rarely surpass 15 per year, analysis shows

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Criminal prosecutions of employers who knowingly hire undocumented workers are rare -- with just 11 people and no companies prosecuted from April 2018 to March 2019, according to an analysis by Syracuse University.

Criminal penalties against employers were enacted by Congress in 1986. The number of prosecutions since then have rarely surpassed 15 per year.

"Not only are few employers prosecuted, fewer who are convicted receive sentences that amount to more than token punishment," the Syracuse researchers said. "Prison sentences are rare."

Cliff Johnson, director of the MacArthur Justice Center at the University of Mississippi School of Law, said it could take months or years for federal agents and prosecutors to review evidence seized from companies to determine whether the employers should face charges.

"Now I suspect there's pressure on this US attorney's office to move quickly due to the high profile of this case and the number of people asking what's going to happen to these corporate defendants," Johnson said.

"But I don't think anyone should read anything into the fact that corporate officials weren't hauled out in handcuffs on the day of the raids."

Affidavits show authorities believe employers knowingly hired unauthorized workers

In affidavits made public after the raids, immigration authorities wrote they had probable cause to believe that the plants in the raids knowingly hired undocumented immigrants, including workers wearing government-issue GPS ankle bracelets while awaiting deportation hearings and others who used the Social Security numbers of dead people.

US Immigration and Customs Enforcement Special Agent Anthony Williams Jr. said in the affidavits that authorities believed the Mississippi plants knowingly hired unauthorized workers with false documents "for the purpose of commercial advantage or private financial gain."

One plant, Peco Foods Inc., said in a statement after the raids that it adhered to "all local, state and federal laws including utilizing the government-based E-Verify program which screens new hires through the Social Security Administration as well as the Department of Homeland Security for compliance."

Another company, poultry processor Koch Foods, said in a statement that it participates in the E-Verify program and strives to ensure its employees are authorized to work in the United States. However, it said workers could have obtained stolen identities from people who are authorized to work in the country and presented them as their own.

"When Koch Foods puts such (workers) through the E-Verify system, the system indicates that the worker is authorized because, unbeknownst to Koch Foods and the E-Verify system, the information that the worker has provided pertains to the stolen identify of an authorized worker," the statement said.

"Federal immigration and discrimination law require Koch Foods to accept documents that appear authentic," the company said.

Immigration worksite enforcement stepped up under President Donald Trump

The company pleaded guilty to felony charges and agreed to pay a criminal fine of $2.5 million, ICE said in a statement.

The company's human resources manager, Jose Humberto Gonzalez, pleaded guilty to a similar federal criminal conspiracy charge, the agency said. He was sentenced to six months of house arrest with electronic monitoring.

Another high-profile case in the state involved a 2008 criminal complaint against the Country Club of Jackson for employing undocumented workers and providing false information to the Social Security Administration. The case was settled with the club agreeing to pay a $214,500 fine and to meet certain vetting and auditing conditions.

Immigration and Customs Enforcement has stepped up worksite enforcement since President Donald Trump took office, conducting a number of large-scale raids at food-processing plants and gardening centers in recent years.

Eldridge said she doubted ranking company officials will face charges after the latest raids.

She said of prosecutors, "I just don't think politically it's going to happen. They're really worried about the economics of the companies but not so much about the economics of the families they've torn apart."