YAKIMA, Wash. -- It's a frequently told story among illegal immigrants: An employer refuses to pay workers after they've finished a job. Fearing deportation, workers say nothing and disappear.

Rarely do such rumors make it any further.

But earlier this week, after a masonry subcontractor said he wouldn't pay employees who were here illegally for their work at the Wal-Mart construction site in West Valley, federal investigators and a nonprofit law firm decided to get involved.

Their stance? Employers must pay employees for hours they already worked, regardless of their legal status.

"That cannot be used as a reason to not pay these people," said Jeannine Lupton, a regional spokeswoman for the U.S. Department of Labor, which has opened an investigation into the company laying the foundation for Yakima's second Wal-Mart store.

The case brings to light what immigrant advocates call a rarely reported trend -- one believed to be growing.

"With jobs so scarce, employers who want to chisel workers out of their wages are in a good place," said Becky Smith, a lawyer in the Olympia office of the National Employment Law Project. "People are afraid to complain about all sorts of conditions at work because jobs are so precious, even awful jobs."

There are no statistics that show how many illegal immigrants are victims of wage theft; It's impossible to track incidents that go unreported.

But on a near daily basis, Smith said, legal aid offices like hers across the country get calls from workers who have not been paid.

"And we've seen a huge uptick in the number of employers who claim that they don't have to pay unauthorized workers," she said. "Of course, immigrant status is no defense to a wage claim. If it were, all employers would hire unauthorized workers and only unauthorized workers."

The nonprofit Columbia Legal Services is investigating the case on behalf of at least one employee who is an illegal immigrant.

A spokeswoman for the state Department of Labor & Industries in Olympia could not confirm whether her agency was also looking into the case. But, Elaine Fischer said, a similar complaint of unpaid wages had been filed against Standard Structures last month.

She said legal status comes up occasionally in wage claims, but "people are afraid to go to a government agency for help if they are not here legally. They may be more inclined to drop the case rather than pursue it."

On Tuesday, Mead Crowell of Arkansas-based Standard Structures acknowledged that he was late in paying about two-dozen employees. He apparently owed the masonry workers three weeks of back pay.

In an interview with the Yakima Herald-Republic, Crowell said he'd recently discovered that some of his employees were illegal immigrants. He said he just learned that through E-Verify -- a federal system employers can use to check whether new hires are legally authorized to work in the United States.

"We can't legally pay these guys unless they are legitimately authorized to work," Crowell said Tuesday.

On Wednesday, he said he would pay all his workers once immigration questions were ironed out with federal officials.

A spokeswoman for U.S. Citizenship and Immigration Services -- the federal agency that operates the E-Verify System -- explained that employers can use E-Verify on a voluntary basis. In Washington, about 1,500 employers use the system on a total of 5,324 sites.

"Our computer is going to look at that information and 96 percent of the time it's going to shoot back to you in a matter of seconds," Sharon Rummery said.

But employers can only use the system within three days of hiring a person, she said. The masonry workers in Yakima had been working at the site for about two months.

"You're not going to run through all your current employees, just your new hires," Rummery said, adding that she could not find Standard Structures in the employer database.

According to the main contractor at the Wal-Mart job site on Nob Hill Avenue, all employees working on the site were believed to be legally authorized workers. Their I-9 paperwork -- apparently some of it fraudulent -- were processed through Wal-Mart's standard procedures, said site supervisor Jack Huff.

On Wednesday, Wal-Mart spokeswoman Michelle Bradford said, "We follow all federal and state laws and regulations regarding verification of identity and work authorization. In turn, we expect our third-party vendors to follow those same laws and regulations."

EDITOR'S NOTE: This story has been revised to correctly identify the federal agency that operates the E-Verify System. It is U.S. Citizenship and Immigration Services.

* Read the U.S. Department of Labor's stance on investigating wage claims by illegal immigrants.