If it becomes law, Arizona's illegal immigration bill that cracks down on employers who knowingly hire illegal immigrants would become the toughest in the U.S.

The bill passed the Legislature last week and would shut down businesses twice caught knowingly hiring illegal immigrants.

Gov. Janet Napolitano has until next Monday to sign it, veto it or let it become law without her signature.

Introduced by state Rep. Russell Pearce, R-Mesa, the law assigns state and county prosecuting agencies to investigate complaints they receive about employers who are making illegal hires.

If the investigation concludes an employer hired an undocumented worker after Jan. 1, 2008, the appropriate county attorney would take the case to Superior Court.

Firms must use a federal database called the Basic Pilot Project to authenticate employees.

That system is designed to match names and Social Security numbers, but it's also been hit by delays and data glitches.

Companies must fire illegal employees within three days of being told and promise not to knowingly hire an undocumented worker again.

Those firms caught violating the law again would lose their licenses.

Dick Walden, a pecan grower with orchards in San Simon and Green Valley, called the measure "anti-business" and "draconian."

Walden and his wife, Nan Stockholm-Walden, said that without a foolproof authentication system, employers like them could be penalized for accepting convincing but illegitimate work papers.

"We could make a mistake with two illegal immigrants, and that would throw into question our entire 60 years of work," Stockholm-Walden said.

Kris Kobach, a University of Missouri Law School professor who helped Arizona lawmakers draft the bill, said that if passed, it would stand in stark contrast to the efforts of the U.S. Senate. "The two strictest state bills we've seen this (legislative) session are the one in Arizona and the one just signed in Oklahoma. I think Arizona's is the strictest," said Kobach.