A worker at an American aeronautics factory went on the rampage yesterday, shooting dead five people before turning the gun on himself.
Eight people were hurt in the attack at the Lockheed Martin plant in Meridian in eastern Mississippi state.
A police spokesman said: "He went through the entire plant, shooting individuals. He appeared to open fire at random on employees." The man, armed with two guns, one a semi-automatic rifle, was reported to have run into the factory buildings at mid-morning and opened fire. Witnesses said he was firing his weapons as if he were in a shooting gallery.
There was no word last night on the attacker's motive or whether he had had grievances with his employers.
John Roberts, the mayor of Meridian, said the killings were "unspeakable". He said the people of the town would stick together in the face of an unexpected tragedy. "We will remain stronger," he said.
Policesaid they were confident the gunman had acted alone. They declined to release his name until the investigation was complete.
Lockheed Martin is the largest defence contractor in the US. The Meridian plant makes parts for the Hercules military transport plane as well as the F-22 Raptor Jet.
The shooting came after police in a suburb of Philadelphia arrested three teenagers armed with rifles, handguns, machetes and 2,000 rounds of ammunition.
Prosecutors said the three were on the brink of launching a scheme to murder three schoolfriends and then randomly shoot other people in the city.
Their investigation was focusing on Matthew Lovett, 18, the leader of the trio, who is said to be obsessed with the film The Matrix. Police have not released the names of the other two because they are 15 and 14.
Mr Lovett, who is being held on $1m (£610,000) bail, was allegedly bent on killing three other teenagers because they had teased him and his younger brother at school. Both were seen as outsiders and the younger brother suffered because of a speech impediment.
People described Mr Lovett as an angry loner. He walked around with a baseball bat, wore black and slicked back his hair in the style of Neo, the main Matrix character played by Keanu Reeves.
On Sunday, the three boys tried to hold up a car at gunpoint, but the driver swerved and drove to the nearest police station. A police officer found the three. They dropped their weapons after the officer drew his gun.
The firearms belonged to Mr Lovett's father, Ron, who was away for the weekend. He raised his sons alone after his wife died 10 years ago.
Ron Lovett insisted yesterday that his son could not have understood the gravity of the alleged plot. "Matt was a wonderful child and he was always protective of his brother," Ron Lovett told NBC's Today show. "I think they were acting out a fantasy and didn't realise what could happen."
He added that they had just graduated from school and were confused about what they would do next.
The prosecutors said yesterday that they would seek to try the three teenagers as adults.