Government audits used to be rare, but that has changed under the Obama administration, which has investigated companies more aggressively, Shusterman said. Government audits of employer I-9 forms rose from 250 in fiscal 2007 to more than 3,000 last year, according to data from U.S. Immigration and Customs Enforcement cited by the Associated Press in December.

Mistakes or missing information, whether intentional or not and which might have nothing to do with a worker being in the country illegally, can lead to stiff penalties. Employers can be fined from $110 to $1,100 per violation for failing to comply with the form's requirements, according to U.S. Citizenship and Immigration Services. Penalties can be even steeper, up to $6,500 per violation, for participating in document fraud, or up to $16,000 for knowingly hiring an unauthorized worker.