A woman who had just been suspended from her job and escorted from a Kraft Foods facility returned with a gun and shot two people dead last night.
A third was critically injured in the shooting in the US city of Philadelphia.
Kraft said in a statement that in addition to the three employees who were shot, a contract worker sustained a less serious injury, but it did not elaborate.
Kraft said the plant would be closed until further notice and the company would provide employees with counselling.
The woman returned to the building in a car 10 minutes after being escorted out and drove through a security barrier before re-entering the building on foot, police said.
As she walked inside, she fired a shot at an employee who had followed her in and had yelled, "Hide, she's got a gun". That shot missed.
The woman then shot the three victims, said police, who didn't know whether they had been targeted. They would not say why the woman was suspended.