They say the contractor manufactured and helped install a water heater in a pump room that's believed to be at the center of the blast.
They're also suing Jacob's Engineering, which they say drew up installation plans.
Federal investigators have said natural gas caused the explosion, and were looking at the possibility a contractor vented a natural gas line to the water heater inside the building.
Three workers died in the explosion. Many more were hurt - some critically. Survivors report smelling gas before the blast.
North Carolina regulators say they are investigating 10 companies in the probe into what happened.
Another company named in the lawsuit - Southern Industrial - has said it did not do any work in the pump room where the explosion allegedly occurred.
Federal investigators say their review of all the facts could take weeks. Many parts of the factory building are still not safe enough to go into.