The hundreds of illegal immigrants recently fired from fast-growing burrito chain Chipotle Mexican Grill Inc had a pretty good run when it came to job security.

Not only did some get jobs with fake Social Security numbers and few questions about their immigration status, in some cases they actually told managers point-blank their papers were no good. And they often stayed on for years.

Marta, an undocumented worker from Mexico, twice used false Social Security numbers to secure positions at the chain now being audited by U.S. Immigration and Customs Enforcement.

She was hired with one false Social Security number in 2003 and rehired as a new employee a few years later under a different name with new, but still fake, number.

The second hire, in 2006, came after Marta told her bosses she needed to change her Social Security number because it belonged to someone else and had caused her mortgage application to fail.

“I thank them (for rehiring me) because I was about to lose my job,” said Marta, who was finally fired last winter for false documents along with about 450 more of Chipotle’s 1,200 Minnesota workers. She asked that her full name not be used for fear of deportation.

Now facing more audits in Washington D.C. and Virgina, Chipotle has become the best-known company in the sights of ICE. Its audit-related firings, including dozens in Washington, are no minor staffing issue.

The 26,500-strong company says it takes the audits, the outcomes and the law very seriously. But Reuters has interviewed eight former employees, most of whom speak fondly of their time at the company, who say Chipotle’s management ignored signs that called workers’ immigration status into question.

The Denver-based company’s woes underscore the challenges employers face with a broken U.S. immigration system. Penalties for non-compliance are paltry and many employers are unable or unwilling to make the effort to ferret out undocumented workers, for whom low-wage jobs are a ticket to a better life.

Analysts want to know how deep the problem runs at Chipotle because massive turnover could raise its labor costs and may threaten its stellar stock.

Chipotle defends its hiring practices and said in an email statement: “We never knowingly hire any employee that is not legally eligible to work in this country and, if we receive any credible information questioning the status of an employee, we look into that and take appropriate action.”

The company known for its motto “Food With Integrity” is more exposed than other chains because it owns all of its 1,100 U.S. eateries, instead of selling outlets to franchisees as many rivals do. It won’t say how many workers it has fired outside Minnesota due to the audits, or whether the hiring of undocumented workers has been limited to audit markets.

CAUGHT OUT

It isn’t hard to find an illegal immigrant working in a U.S. restaurant: estimates of the percentage of illegal workers in the food service industry range from around 10 to as much as 40.

While many chains rely on them, Chipotle was the highest profile company caught when immigration enforcement strategy switched to cracking down on employers rather than workers in 2009.