Immigration authorities had bad news this week for American Apparel, the T-shirt maker based in downtown Los Angeles: About 1,800 of its employees appeared to be illegal immigrants not authorized to work in the United States.

But in contrast to the high-profile raids that marked the enforcement approach of the Bush administration, no federal agents with criminal warrants stormed the company’s factories and rounded up employees. Instead, the federal immigration agency sent American Apparel a written notice that it faced civil fines and would have to fire any workers confirmed to be unauthorized.

The treatment of American Apparel, which has more than 5,600 factory employees in Los Angeles alone, is the most prominent demonstration of a new strategy by the Obama administration to curb the employment of illegal immigrants by focusing on employers who hire them  and doing so in a less confrontational manner than in years past.

Unlike the approach of the Bush administration, which brought criminal charges in its final two years against many illegal immigrant workers, the new effort makes broader use of fines and other civil sanctions, federal officials said Thursday.