Foreign automakers have received billions of dollars in subsidies and tax incentives to build factories in the U.S. and create American jobs.

A new CBS News primetime series, "CBSN: On Assignment," uncovers the hidden foreign workforce being used to build sections of U.S. auto plants. The show premieres on CBS on Monday, July 31 at 10 p.m. EST & PST, streaming simultaneously on CBSN, the network's streaming news service.

This CBSN investigation took us thousands of miles, from South Carolina to Slovenia, in eastern Europe, in search of answers.

For three years, Gerald Greiner managed safety on American construction projects for a German contractor called Eisenmann. His first job was at Mercedes in Vance, Alabama in 2013.

"There was Polish and Slovenian and Croatian people there," he told correspondent Vladimir Duthiers. "It was hard for me to believe because I just didn't understand why they would be here."

What did they do? "Anywhere from steel erection to pipe fitting to pouring concrete to installing equipment, just about everything."

The cars would be built by American workers -- but the building of the auto plants was being done by foreign workers. "Exactly," said Greiner. "They come in at groundbreaking, they're done at start of production."

Duthiers asked, "Did you think to yourself that the jobs that these guys were doing could be done by Americans?"

"Oh yeah, absolutely. Yes."

Our investigation led us to an apartment complex in Spartanburg, South Carolina, where it appeared workers from Slovenia and Croatia were being housed by their employer.

A "CBSN: On Assignment" producer witnessed foreign workers being picked up and taken to a BMW plant under construction in South Carolina. CBS News

Around 6:30 a.m., the workers had one last cigarette, before a van came to take them to work.

A CBS producer filmed the van of workers going through BMW security at 6:45 a.m. The producer shot 15 or more vans and vehicles filled with workers from Eastern Europe.

"I'm angry. Angry," said Daniel Travancic, who has worked for the Local 104 sheet metal workers union out of San Jose, California, for more than a decade.

"There's lots of guys out there still looking for work in the United States," he said. "And now we have how many thousands and thousands of East European workers working here, and they're abused, too? Who lets this happen?"

BMW, Mercedes, Volvo, Tesla, Volkswagen and Eisenmann declined CBS News' request for an interview. But they sent statements that all said essentially the same thing: their contractors are legally obligated to comply with all immigration, safety and employment laws, and any violations are promptly addressed.

Eisenmann also told CBS News they stopped writing letters to the U.S. Consulate on behalf of specific employees' visa requests last summer.

Last week, Mercedes said they are investigating allegations of B1/B2 visa abuse and have already banned one subcontractor from the Alabama work site as a result. As for a different subcontractor, Ism Vuzem, they told CBS News they were on vacation and would get back to us after the summer.