It's not perfect. The E-Verify system lets unauthorized workers slip through its filter more than half the time because it cannot spot identity theft, according to a federal report.

The system, a federal database available to employers, failed to identify those workers as unauthorized 54 percent of the time, according to an audit released last year. Investigators discovered that was the percentage of illegal workers incorrectly deemed eligible for work.

The study done for U.S. Citizenship and Immigration Services found that overall, the system correctly determined a worker's immigration status 96 percent of the time.

Researchers wrote that the results were "not surprising" given that many unauthorized workers use someone else's identity to get a job.

"E-Verify, as currently formulated, does not detect most identity fraud cases for workers who use information about real employment-authorized persons," the report said. "If a worker presents documents that contain information about a real work-authorized person, and if the documents appear to be valid, E-Verify is unlikely to detect the identity fraud."

About 242,000 businesses across the country are registered with the system. Last year, more than 583,000 queries from state businesses were run through the system, more than double the total of the previous year.