It was unclear whether the gunman, identified as Timothy Hendron of Webster Groves, Mo., was among the fatalities, KMOX Radio, St. Louis, reported.
Hendron, a disgruntled employee who was reportedly involved in a lawsuit against the company over financial losses, allegedly opened fire at the ABB plant at 6:30 in the morning.
Around 11 a.m., the body of a man thought to be Hendron was removed from the scene. However, it was unclear whether the man had died from a self-inflicted gunshot wound.
Information has been difficult to confirm, Swiss engineering company ABB said in a statement.
This is obviously a very serious situation and we are working to gather more information as it becomes available, the company said.