LOS ANGELES — Immigration agents descended on dozens of 7-Eleven convenience stores before dawn Wednesday to begin checking on employees’ immigration status in what officials described as the largest operation against an employer under Donald Trump’s presidency.

Agents targeted about 100 stores nationwide, broadening an investigation that began four years ago with a case against a franchisee on New York’s Long Island. The audits could lead to criminal charges or fines over the stores’ hiring practices. Twenty-one people suspected of being in the U.S. illegally were arrested.

The action appears to open a new front in Trump’s expansion of immigration enforcement, which has already brought a 40 percent increase in deportation arrests.

Derek Benner, a top official at U.S. Immigration and Customs Enforcement, said Wednesday’s operation was “the first of many” and “a harbinger of what’s to come” for employers.

“This is what we’re gearing up for this year and what you’re going to see more and more of is these large-scale compliance inspections, just for starters,” said Benner, acting head of ICE’s Homeland Security Investigations, which oversees cases against employers.