When federal agents raided dozens of 7-Eleven stores across the country this month and arrested 21 workers suspected of being in the country illegally, U.S. Immigration and Customs Enforcement acting Director Tom Homan declared that the raids were meant to send a message to employers: "If you are found to be breaking the law, you will be held accountable."

But after all the smoke from the day's fiery rhetoric cleared, one huge question remained: How did these workers get hired in the first place?

Renewed calls on Capitol Hill came to make a program called E-Verify mandatory for all businesses — and many wondered why the program allows so many to slip through the cracks.

E-Verify is a 21-year-old electronic program designed to filter out undocumented immigrants who apply for jobs. But many experts say it does a poor job of distinguishing whether a Social Security card is valid. First, few companies are audited. Second, currently only those who have been caught employing illegal residents must use E-Verify in most states.

With such a low chance of being audited, E-Verify is "a wink and a nod from the government to employers," said Daniel Costa, director of immigration law and policy research for the Economic Policy Institute, a pro-labor think tank based in Washington, D.C.

An estimated 763,500 employers were taking part in E-Verify as of Jan. 16, according to U.S. Citizenship and Immigration Services, which oversees the program. That sounds like a lot, but there are more than 18 million businesses in the U.S.

In the last fiscal year, 98.88 percent of the employees entered into the E-Verify system were cleared to work, according to USCIS.

But some groups trying to put more restrictions on immigration say E-Verify isn't as flawed as critics make it seem and have called on federal officials to expand the program. The idea that somebody could put down a false or stolen Social Security number and get hired by an employer that uses E-Verify is "impossible" to believe, said Joe Guzzardi, spokesman for Progressives for Immigration Reform.

U.S. immigration authorities say they have gradually improved the system over the years to fix some of its flaws and to close loopholes, for example, adding a photo tool and more sophisticated programs to compare the information to that provided for passports, driver's licenses and other ID.

Sanchez writes for The (San Jose) Mercury News.