According to Jorge Mujica of UE, who led the walkout, some workers were verbally notified that they had 10 days to show proof of their legal working status or they would be fired. Workers claim the employer began firings on the basis of outdated "no match" letters sent to the company by the Social Security Administration, Mujica said.

Employers or employees can receive such a letter if a Social Security number does not match up with W-2 forms submitted by the employer. But mistakes could come from typographical errors, a name change or incomplete records, according to the Social Security Administration.

Mujica, however, said no employees have received "no match" letters from the SSA and are legal to work.

Workers like Jesus Rodriguez, who has been an employee for two years, says he and his co-workers should not be required to show documentation.

"They are not respecting our rights," he said in Spanish.

But Evelyn Marsh, an immigration law attorney in Chicago, said depending on what documentation is in question, the company may be within its legal rights. Employers, she said, "are somewhat limited, but once they have this employer-employee relationship, there's no contract."

Ballco Manufacturing Company was started by three Aurora brothers of the Szilage family around 1960 and was sold in 1989. It is among the nation's top manufacturers of valve components.

Under the Immigration Reform and Control Act, employers must verify worker eligibility within three days from the start of employment.

Some picketing workers said they have held jobs for six years or longer.