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. Federal agents will concentrate on businesses employing large numbers of workers suspected of being illegal immigrants, the officials said, and will reserve tough criminal charges mostly for employers who serially hire illegal immigrants and engage in wage and labor violations.

"These actions underscore our commitment to targeting employers that cultivate illegal work forces by knowingly hiring and exploiting illegal workers," said Matt Chandler, a spokesman for the Department of Homeland Security.

On Wednesday, Immigration and Customs Enforcement, the federal agency known as ICE, said it had sent notices announcing audits of hiring records, like the one it conducted at American Apparel, to 652 other companies across the country. Officials said they were picking up the pace of such audits, after performing 503 of them in 2008.

The names of other companies that received notices have not been made public. American Apparel became a window into the new enforcement tactics because, as a publicly traded company, it issued a required notice on Wednesday about the hiring audit.

The Obama administration's new approach, unveiled in April, seems to be moving away from the raids that advocates for immigrants said had split families, disrupted businesses and traumatized communities. But the outcome will still be difficult for illegal workers, the advocates said, who will lose their jobs and could face deportation.