The slayings occurred in the early hours of Sunday morning in a house in Crandon, in the state of Wisconsin, at around 3:00 a.m. local time (8:00 a.m. GMT), when the gunman opened fire at a house where local youths had gathered to "eat pizza and watch movies."
The gunman was identified as 20-year-old Tyler Peterson. Local Police Chief John Dennee said that Peterson worked full time for the Forest County Sheriff's Office and part time as a Crandon police officer. He was not on duty at the time of the massacre.
Preliminary reports indicate that Peterson launched his attack after an argument with his 17-year-old girlfriend. The killings have been described as a "crime of passion" by local U.S media.
At least three of the dead were high school students. Another of the victims, Lindsey Stahl, was 14. A seventh youth is currently in a critical condition in hospital.
Crandon has a population of around 2,000 people, and is about 225 miles (360 kms) north of Milwaukee.
It is as yet unclear if the gun used in the murders was Peterson's service weapon.
The right to "keep and bear arms" is enshrined in the Second Amendment of the U.S Constitution.