CHICAGO (AP/CBS) —  Five Chicago area 7-Eleven stores were part of a nationwide operation, in which U.S. immigration agents descended on locations before dawn Wednesday to open employment audits and interview workers in what officials described as the largest operation against an employer under Donald Trump’s presidency.

Agents targeted about 100 stores across the United States.  The audits could lead to criminal charges or fines over the stores’ hiring practices.

An official with the Immigration and Customs Enforcement would not give locations of the Chicago stores, citing “operational and investigative reasons.”

The action appears to open a new front in Trump’s sharp expansion of immigration enforcement, which has already brought a 40 percent increase in deportation arrests and plans to spend billions of dollars on a border wall with Mexico. Hardliners have been pressing for a tougher stance on employers.

Derek Benner, a top official at U.S. Immigration and Customs Enforcement, told The Associated Press that Wednesday’s operation was “the first of many” and “a harbinger of what’s to come” for employers. He said there would be more employment audits and investigations, though there is no numerical goal.

7-Eleven Stores Inc., based in Irving, Texas, with more than 8,600 stores in the U.S., didn’t immediately respond to a message seeking comment.

Though agents arrested 21 people suspected of being in the country illegally during Wednesday’s sweep, the action was aimed squarely at management.

Illegal hiring is rarely prosecuted, partly because investigations are time-consuming and convictions are difficult to achieve because employers can claim they were duped by fraudulent documents or intermediaries. Administrative fines are discounted by some as a business cost.

Trump is pursuing “its own kind of unique strategy” tied to its broader emphasis on fighting illegal immigration, including enforcement on the border, Benner said. Some workers may get arrested in the operations but authorities are targeting employers because they are job magnets for people to come to the country illegally.

“We need to make sure that employers are on notice that we are going to come out and ensure that they’re being compliant,” Benner said “For those that don’t, we’re going to take some very aggressive steps in terms of criminal investigations to make sure that we address them and hold them accountable.”

Wednesday’s operation resulted from a 2013 investigation that resulted in charges against nine 7-Eleven franchisees and managers in New York and Virginia. Eight have pleaded guilty and were ordered to pay more than $2.6 million in back wages, and the ninth was arrested in November.

In the 2013 investigations, managers used more than 25 stolen identities to employ at least 115 people in the country illegally, knowing they could pay below minimum wage, according to court documents. The documents say 7-Eleven corporate office does automated payroll, requiring franchisees provide employee names and Social Security numbers to pay workers through direct deposit or check.

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