GREELEY, Colo. -- Last Friday, just days after federal agents raided six Swift & Co. meat-processing plants and carted away nearly 1,300 workers as illegal immigrants, company officials scrambled to find replacements for vacancies on the killing floor.

At a city job center here near the company's corporate headquarters, one prospective new hire came from Somalia. He had lived in the U.S. for four months and barely spoke English. He didn't say how he had arrived in the U.S. or whether his presence was legal. But through an interpreter he told his interviewers that he was happy to work with knives. Then he produced a crisp new Social Security card to show he was ready to start, and Swift offered him a job.

While Swift itself wasn't charged with any wrongdoing as a result of the government raids, after its brush with the law, it might seem like a risky move to hire a recent immigrant without further background checks.

But Swift says that thanks to recent dealings with federal officials and its own efforts to refine its hiring practices, it thinks it has figured out how to reduce its chances of being duped. If a prospective new hire isn't who he or she says, company officials say, asking a few simple questions about where the job applicant lived and worked, and where he or she obtained an identity card, will help them to spot the lie.

Swift isn't the only company trying to figure out how to cope with a tough new federal enforcement policy against employing illegal aliens. Officials from the Department of Homeland Security's Immigration and Customs Enforcement bureau say that since last week's raids on the six Swift plants, they're being swamped with calls from companies seeking advice on properly screening their work forces. Some officials say the Swift raid -- the largest immigration bust ever mounted against a single company -- may forever change the way companies hire unskilled foreign workers.

"The action should send a clear message to employers: Hiring illegal workers is not acceptable," says ICE Director Julie Meyers.

But there are concerns among businesses, immigration lawyers and trade unions that the government's action could turn businesses into immigration enforcement agents, and there is little government guidance about what employers should look for or do with foreign-born workers. That leaves the whole process to guesswork.

"If the government believes that there are things that companies should be doing but isn't telling companies what they are, it creates an unfair playing field for an employer," says Tom Green, an immigration lawyer in Washington, D.C. "If the government believes there are signposts a business should be looking out for, they must speak up. This 'gotcha' approach is the last thing we need."

Swift's program is itself the product of corporate conjecture. "We had to go out on our own without any government guidance and figure out what to do," says company spokesman Sean McHugh.

Even before ICE agents began investigating Swift, the company realized it had a problem. Since 1986, federal law has required all employees to fill out an I-9 Employment Eligibility Verification form. The form says in big, bold letters that it's up to the employee to produce one of 10 acceptable documents -- a passport, driver's license or Social Security card, for example -- to establish identity and employment eligibility. If an employer asks for more identification, it can be sued. That happened to Swift four years ago when the company asked Hispanic job applicants to provide extra proof of their legal status. The company was fined $200,000 for employment discrimination.

Even before that, in 1998, the company had volunteered to participate in a pilot worker ID program set up by the government to ensure employers that they were hiring legal workers. The program, which now has 12,000 participants, checks a new hire's identity documents against the Social Security Administration's database. If a worker's name and Social Security number don't match -- a giveaway that the number is fake -- the information is reported to Homeland Security for further investigation.

But according to congressional investigators, federal officials have known for at least a year that the program doesn't detect when documents have been stolen, allowing someone to assume another's identity, and that stolen documents have increasingly been used to defeat the system.

With illegal immigration growing as a political issue, ICE announced last year that it planned to take a tougher stance against employers. Knowing that its industry was a magnet for unskilled foreign labor, Swift in February hired private consultants for help. It needed to figure out a way to screen workers that steered clear of antidiscrimination laws.

But about the same time, Marshalltown, Iowa, police warned ICE that many workers in the company's plant there weren't whom they claimed to be. The government subpoenaed I-9 forms for an audit. It reviewed 2,100 and kept 648 for further study.

The company quickly took notice. "What is it about these 600 forms that we were missing?" James Hamilton, a Swift human-resources executive, says he wondered.

The company eventually figured out that ICE was looking for people with recently issued ID cards and Social Security cards who claimed to be citizens but had little or no work, education or residence history -- or had histories that didn't match the places where their Social Security cards and IDs had been issued.

Starting in October, Swift used this information to design a process to help job interviewers spot new ID cards and other red flags. "We have implemented this process, and we been quite successful with it," Mr. Hamilton says. "We have terminated or rescinded job offers of 1,500 people since July."

But that didn't stop last week's raid -- and in fact may have hastened it. Swift fired about 400 employees from its Marshalltown plant under the new process. ICE already had the plant under investigation, and when officials heard about the terminations and the fired employees that had fled along with their fake personas, they ordered the company to stop what it was doing. They then told Swift's lawyers that ICE was planning to raid all the company's facilities on a single day to interview and possibly detain illegal workers to prevent further flight. The company, saying that such a raid could cost it millions of dollars in lost production, tried and failed to get a court order to stop the raid.

While ICE officials have praised Swift for cooperating on the day of the raid, they have also suggested that firing employees at a time when the company was under investigation may have been an attempt to thwart enforcement action. Swift denies the charge and has produced letters (which ICE confirms are authentic) in which investigators said that the company was allowed to manage its own work force as long as it didn't base its actions solely on the fact that the government had retained some I-9 forms.

Now Swift is using its new process only to screen new hires like the young Somali.

"The offer we gave him is conditional on him passing a drug test, a physical exam and making it through the I-9 screening process," says Mr. Hamilton. "It will still take a few days and some careful scrutiny before we put that knife in his hand."