A gunman armed with an assault rifle has killed three people and injured four more at a factory in Missouri.

Workers at the ABB Power factory are hiding in offices and on the roof awaiting rescue. The plant has been sealed off and local roads closed.

US correspondent Bill Manson says the gunman is still at large, and is believed to be 51 year-old Timothy Hendron. He says Hendron's neighbour Ronald Bartram is finding it hard to believe his friend would do such a thing and has even questioned whether police are looking for the wrong man.

Bill Manson says Hendron is a hunter and owns a number of weapons.