Employers with questions can visit ICE's Web site at www.ice.gov or call 866-DHS-2ICE for answers, she said.

"This rule went through the federal rule-making process" starting last June, Reilly said. It was amended after a 60-day public comment period. "All the answers are out there."

Of about 250 million Social Security numbers used for employment, about 4 percent are mismatches, Reilly said. A mismatch can happen for a variety of reasons, such as when women marry but don't report their new name to Social Security or because of a typo.

When employers receive a mismatch letter, they'll have 30 days to check their records to ensure the mismatch wasn't an employer error. If it isn't an employer error, then the employee has another 60 days to clear up a discrepancy with Social Security. If that can't be done, the employee has three days to fill out a new I-9 employment eligibility form, without using the incorrect Social Security number. If the process doesn't confirm a worker's eligibility to work legally, then the employer must terminate the worker.

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E-mail: dbulkeley@desnews.com