Immigration agents descended on Chipotle Mexican Grill restaurants on Tuesday, interviewing employees in about two dozen restaurants in Los Angeles, Atlanta and other cities.

U.S. Immigration and Customs Enforcement (ICE) agents also interviewed workers in Minnesota and Washington, D.C., said Robert Luskin, Chipotle's outside counsel and a partner at Patton Boggs in the capital.

Luskin said ICE gave Chipotle enough advance notice of the interviews by plain-clothed agents that the company had the opportunity to send a note to employees telling them it wanted them to cooperate.

Roughly 500 undocumented workers have been fired as a result of ICE audits of the chain's hiring paperwork in Minnesota, Virginia and Washington, D.C., and Tuesday's move by ICE suggests the government interest could be expanding.

Chipotle co-Chief Executive Monty Moran said on April 20 that the criminal division of the U.S. Attorney's Office in Washington, D.C. requested documents related to the ICE audits.

That request came hours after a Reuters story quoting fired immigrant workers who said that Chipotle -- one of the highest-profile employers to fall under the scrutiny of ICE -- ignored signs that pointed to the illegal status of some of its workers.

We've got nothing to hide, Chipotle attorney Luskin said. We're absolutely convinced that nobody did anything wrong.

He added that Tuesday's action by ICE doesn't signal a broader or more serious or more substantial investigation and that he had no reason to expect that the investigation would be confined to audit areas.

ICE spokeswoman Cori Bassett did not respond to requests for comment.

ENFORCEMENT SHIFT

U.S. immigration enforcement has shifted considerably in recent years. Notably, the Obama administration is cracking down on employers rather than illegal workers.

Carl Shusterman, a former prosecutor for ICE's predecessor agency, the U.S. Immigration and Naturalization Service, said interviews like the ones ICE conducted at Chipotle on Tuesday show how the administration is ratcheting up pressure. He also acknowledged that the tactic could scare off undocumented workers.

I would imagine what happens is that the people who are illegal will never come to work again, said Shusterman, who now is in private practice in Los Angeles.

Chipotle has won plaudits from Wall Street for its seemingly uncanny ability to hold down labor costs -- a major factor behind its six-fold increase in share price since late 2008, in the depths of the recession.

The immigration probe -- as well as the simultaneous criminal investigation -- into its hiring practices may inflate costs in the long run if it leads to more mass firings. That could be bad news for Chipotle, which like other restaurants, now is grappling with rising prices for everything from beef to produce.

It is also a blow to the reputation of a restaurant chain that prides itself on serving Food with Integrity.

Should the investigations uncover widespread abuse of immigration laws, co-Chief Executives Steve Ells and Monty Moran could face criticism for allowing the practice to spread through the company's roughly 1,100 U.S. restaurants. Unlike many rivals, Chipotle owns and operates its restaurants and is ultimately responsible for hiring.

The U.S. fast-food industry historically has offered relatively low pay and paltry benefits to legal workers and, as a result, has struggled with high employee turnover.

Experts say restaurant owners are attracted to illegal laborers because they work hard, are loyal and will go the extra mile to hold down a job.

It is hard to know the extent of hiring of illegal immigrants in restaurants. But immigrants -- both legal and illegal -- account for about a quarter of workers in the restaurant and food services industry and their numbers have climbed in recent years.

(Editing by Steve Orlofsky)