SIX people have been killed after an off-duty police officer opened fire at a house in the midwestern United States.The gunman, named as 20-year-old Tyler Peterson, was later killed by a police sniper.

SIX people have been killed after an off-duty police officer opened fire at a house in the midwestern United States.

The gunman, named as 20-year-old Tyler Peterson, was later killed by a police sniper.

It is believed the part-time officer with the Forest County Sheriff's office may have opened fire at the house in Crandon, north Wisconsin shortly before 3am on Sunday, following an argument with his girlfriend.

A seventh victim, described as a youth like the others involved, remains in hospital in critical condition.