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When federal agents raided dozens of 7-Eleven stores across the country earlier this month and arrested 21 workers suspected of being undocumented immigrants, U.S. Immigration and Customs Enforcement director Tom Homan declared that the highly publicized 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 undocumented immigrants get hired in the first place?

At a time when the national debate over immigration is at its tipping point, questions have begun to resurface about the effectiveness of E-Verify, a 21-year-old electronic program designed to filter out undocumented immigrants who apply for jobs. Some conservative Republicans in the House indicated on Tuesday that they intend to make E-Verify a key issue as Congress in the next few weeks vigorously discusses the future of immigration, including the “Dreamers,” the 800,000 young undocumented immigrants who were illegally brought to the U.S. when they were children.

Many Americans wonder how millions of undocumented immigrants so easily slip through the job-authorization system even though it’s been three decades since employers have been asked to verify the immigration status of job applicants.

After all, credit card companies nearly seamlessly verify tens of millions of 16-digit numbers every day with a tiny little machine that easily fits on the counter of nearly every store in America. What could be so hard about verifying that every job applicant has a valid Social Security number or “alien registration number” that indicates that the jobseeker is a legal U.S. resident even if he or she is not yet a citizen.

Some immigration experts contend that E-Verify sounds like a no-nonsense solution but that it’s essentially a political fig leaf, with so many significant flaws and loopholes that it allows employers to continue to knowingly hire undocumented workers with few repercussions.

Alex Nowrasteh, an immigration policy analyst at the Cato Institute’s Center for Global Liberty and Prosperity, a libertarian think-tank in Washington, D.C., said the E-Verify system accepts passports, Social Security cards and other documents submitted by job applicants even if they are fraudulent — which is the way most undocumented immigrants secure employment.

Nowrasteh said “a large portion of employers still don’t use it for new hires” because they know that government audits of traditional paper forms are so few and far between.

Daniel Costa, director of immigration law and policy research for the Economic Policy Institute, a pro-labor think tank based in Washington, D.C., agreed with that assessment. He noted that even though audits of the I-9 forms soared during the Obama administration, an average of only 2,000 workplaces — including some enrolled in E-Verify — were audited annually during the eight years Barack Obama was president.

According to U.S. Citizenship and Immigration Services, which oversees the E-Verify program, an estimated 763,500 employers were taking part in the program as of Jan. 16.

That sounds like a lot, but it’s only a tiny percentage of the more than 18 million businesses in the U.S.

The reason for the low participation rate is that the program is voluntary in most states. It’s mandatory in most of the U.S. only for businesses that have been caught hiring undocumented immigrants.

The Social Security Administration estimated that 1.8 million immigrants were working with fake or stolen Social Security cards in 2010 — and that number is projected to reach 3.4 million by 2040.

Companies that don’t use E-Verify require jobseekers to fill out the I-9 forms to check work eligibility. The employers then must certify that the documents provided by the applicant appear to be genuine.

With such a low chance of being audited, Costa said, the whole job-authorization system is “a wink and a nod from the government to employers” that lets them continue to hire workers who are not in the U.S. lawfully.

The Cato Institute’s Nowrasteh argues that E-Verify exists simply because it allows politicians to claim the U.S. is being tough on immigration without actually having to be tough.

“Nobody wants to shut down businesses. That’s expensive politically and economically,” he said. “It’s much easier to have a system that doesn’t work (but which) sounds like a silver bullet.”

Calls for “mandatory E-Verify” — particularly among immigration hardliners in the Republican Party — have been increasing in recent weeks as Congress wrestles with the “Dreamer” issue. And polls have shown that more than two-thirds of the American public believe the E-Verify should be mandatory.

Capitalizing on that sentiment, Rep. Lamar Smith, a Texas Republican who sits on the House Judiciary and Homeland Security committees, recently introduced a measure to require all employers to use the program. “By expanding the E-Verify system to all U.S. employers, this bill will ensure that jobs only go to legal workers,” Smith said.

Groups trying to put more restrictions of 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, an organization in Washington, D.C., that says it’s fighting for more immigration restrictions because it will benefit American workers. The group wants to make E-Verify mandatory.

U.S. immigration authorities say they’ve gradually improved E-Verify over the years to fix flaws and close loopholes.

Sharon Rummery, a spokeswoman for U.S. Citizenship and Immigration Services, said when it became known that people could get around E-Verify by taking on stolen identities, the agency in 2007 added a “photo tool” feature that allows employers to enter new hires into the system to check the photo on the identification card they used and compare it to the photo of the person who was originally granted the green card or work authorization document.

Under E-Verify, once employees provide documents to prove they’re living in the U.S. legally, employers use a device that looks a lot like the credit-card machines that stores use. A software program then runs the information on the documents through U.S. Department of Homeland Security and Social Security Administration databases to confirm whether the job applicant is here legally.

If the information clears the system, the employer will receive an “Employment Authorized” message almost immediately. If the job applicant isn’t able to work in the U.S., the employer will receive a “tentative non-confirmation” message.

The employer is then required to notify the employee, who is given a brief window of time to fix the issue. Those who are unable to provide documentation are not allowed to work.

In the fiscal year that ended Sept, 30, 98.88 percent of the employees entered into the E-Verify system were cleared to work, requiring no employee or employer action, Citizenship and Immigration Services says.

Rep. Zoe Lofgren, a San Jose Democrat who is the ranking Democrat on the House’s Immigration and Border Security Subcommittee, said E-Verify has a fairly high accuracy rate, but the underlying problem is the country’s “completely broken” immigration system.

She said employers may now have a reliable way to check if their workers are in the U.S. legally, but they know that they need those same workers to keep their businesses afloat.

“You go into the fields in Salinas and you have agricultural workers picking the crops. You could use E- Verify and find out that half of them or more are undocumented,” Lofgren added. “So then who picks the crops? That’s the problem.”

E-Verify by the numbers

To confirm a job applicant’s eligibility to work in the U.S., the E-Verify program compares information provided by a job applicant with data in Department of Homeland Security and Social Security Administration computers.

10.5 million – Number of queries run through the E-Verify system so far in the 2018 fiscal year, which began Oct 1.

763,500 – Employers registered for E-Verify as of Jan. 16.

2.5 million — Number of hiring sites represented by E-Verify.

1,316 — Average number of employers who have registered for the program weekly so far in fiscal year 2018.

Source: U.S. Citizenship and Immigration Services.