(CNN) Fiat Chrysler called in outside investigators after a noose was found last month hanging inside a Ram truck plant in Michigan, the company told CNN.
But despite inquiries by internal and external teams at the Sterling Heights plant near Detroit, the culprit could not be identified, it said.
"If and when that person is identified, their relationship with the company will be terminated. FCA will continue to conduct focused training to underscore the value of diversity and inclusion. This type of behavior will simply not be tolerated," Fiat Chrysler Automobiles said in a statement.
It's the second investigation into racism at the plant in two years.
FCA would not give details but said the first incident, in 2017, was resolved after an internal inquiry.
United Auto Workers (UAW) Vice President Cindy Estrada, who oversees Fiat Chrysler for the union, said: "We are appalled at these reports."
"It is always unacceptable when a member suffers the indignity of slurs based on race, gender, religion or sexual orientation," she said. "The UAW has been and will always be a leader in fighting discrimination in the workplace."
Fiat Chrysler is not the only automaker to have seen workers targeted by hate.
The UAW took up the issue and passed a resolution this year that said it would demand bans on workplace discrimination and training on anti-discrimination practices in its negotiations with employers.
"As we strive to build an inclusive union, we demand that employers make a similar effort and dedicate the necessary resources to ensure our workplaces are inclusive," it said.
Other companies are being sued for their handling of racist incidents at their workplaces, including the hanging of nooses.
Five nooses appeared in General Motors' Toledo Powertrain plant in Ohio between 2017 and 2018 as a threat to black supervisors and workers, along with racial slurs written on bathroom walls and verbal threats, according to a lawsuit filed in November.
Two of the plaintiffs told CNN of the abuse they said they suffered and how it ended up driving them from their jobs.
GM did not dispute the incidents but said it has and will take action.
In a tweet after CNN's story published, GM said it was "outraged that anyone would be subjected to racist behavior. We have zero tolerance for discrimination -- this is not who we are. We're working to drive this out of our workplaces."
It also offered a $25,000 reward for help in finding the culprits for the noose hangings.
A short distance from Toledo, in Maumee, Ohio, a black worker told CNN he found a noose hanging over his area at a UPS center.
According to a lawsuit filed this month by 19 current and former workers, that was one of a string of racist or discriminatory incidents at the company.
UPS did not dispute a noose was hung but said the culprit was promptly fired and said it takes any form of racial discrimination extremely seriously.