The union says Duke Energy used a contractor, Sunbelt Insulation, based out of Greer, South Carolina.
And according to the IBEW, that company hired illegal immigrants more than a hundred of whom worked for Duke Energy in five different power plants in Ohio, Indiana and Kentucky at various times over the past few years.
The union says they have documents, including recordings of conversations which were gathered by the International Association of Heat and Frost Insulators union.
These conversations, they say, show that Sunbelt Insulation not only employed illegal immigrants who ended up working in Duke power plants, but purchased fake social security cards and falsified I-9 documents for those workers.
"We've got people in this area who want jobs," said IBEW Local 1347 Business Manager Stephen Feldhaus.
"That's what we're upset about," said Feldhaus. "And then the fact that you've got illegals in there and you have no background checks, they're not doing any background checks. So it's putting our members' safety at risk."
Duke Energy says they've done an internal investigation and had a third party investigate the matter.
They say those investigations showed that Sunbelt Insulation did have the proper documents for their workers.
"Everything that we require based on the federal guidelines was completely in order, we were very pleased with the outcome," Duke spokesperson Johnna Reeder said about the investigations.
"However, we did turn it over to the federal authorities who will then in turn decide if everything followed the proper documentation, and if there were any discrepancies," said Reeder.
Both Duke Energy and the IBEW say they've provided all of their documentation to the Immigrations and Customs Enforcement (ICE) office.