A man about to go to prison for stealing engine parts from the factory he worked in for nearly 40 years forced his way into the plant near Chicago and shot dead four people before killing himself.
Four others were wounded, two critically, in the latest of a plague of violent attacks at work, in schools and in other public places in the United States.
Police said Mr William Baker (66) showed up at the plant with an AK-47 assault rifle, a pump-action shotgun, a hunting rifle and a revolver stashed inside a golf bag.
He forced his way past a security guard and walked up and down aisles on the factory floor shooting randomly. He ended up in an office where he shot his final victim and then himself.
"It could have been a lot worse," said Melrose Park Police Chief Mr Vito Scavo, citing the large amount of ammunition Mr Baker carried.
The attack occurred at an engine manufacturing plant operated by International Truck and Engine Corp, a subsidiary of Navistar International Corp, in Melrose Park just west of Chicago.
Mr Baker worked as a forklift operator at the plant from 1955 until 1995 when he was sacked after the FBI broke up a theft ring.
He was indicted in September 1999 along with five others for conspiracy to commit theft, according to a spokeswoman for the US Attorney's office in Chicago.
The spokeswoman said Mr Baker had pleaded guilty to the federal charge in November 2000 and had been sentenced to five months in jail followed by five months home confinement. She said he was scheduled to report to prison later today.
Officials said Baker also had a sex offence record.
Reuters