The business is located in the Chicago suburb of the Aurora..
All the five employees who died in the shooting were men, police said.
The shooter was killed during an exchange of gunfire with law enforcement officials, Aurora Police Chief Kristen Ziman said.
John Probst, an employee who survived the shooting, told CNN affiliate WLS that the gunman was a coworker.
He was "running down the aisle" with a pistol that had a green laser sight on it, he said.
"As soon as I saw the green thing and heard the shots, we left," said Probst, who's worked at the plant for 40 years.
"He started opening up on the room and he was just shooting everybody."