Six people who worked on construction of a North Dakota gas plant have filed a federal lawsuit alleging racial discrimination.

The Bismarck Tribune reports that the six workers, all of whom are black, are suing a California-based construction company for incidents that allegedly happened between 2012 and 2014. They say they were victims of racially offensive graffiti and harassment and unfair employment practices and work assignments.

The suit says the workers were denied raises and promotions, were forced to perform dangerous work in freezing temperatures and were not permitted to take mandatory breaks in "warming trucks."

The complaints were originally filed with the Equal Employment Opportunity Commission, but lawyers for the workers say negotiations broke down.

A lawyer for the construction company, KS Industries, did not return repeated phone messages.