Peterson was shot to death after opening fire early Sunday on his ex-girlfriend and a group of friends who had gathered for pizza and movies on their high school's homecoming weekend.
Peterson was off-duty from his full-time job as a Forest County deputy sheriff; he also was a part-time Crandon police officer.
Another teen was critically wounded.
The victims include Peterson's reported former longtime sweetheart Jordanne Murray, 18, Katrina McCorkle, 18, Leanna Thomas, 18, Bradley Schultz, 20, Aaron Smith, (age not available)and Lindsey Stahl, 14, according to the Milwaukee Journal Sentinel.
Charlie Nietzel, 19, of the neighboring town of Pickerel was wounded. He was in critical condition late Sunday at a local hospital.
Peterson would later end up dead, shot by the Crandon SWAT team.
David Franz, 36, who lives with his wife two houses from the duplex where the shooting occurred, said it was hard to accept that someone in law enforcement was the gunman.
"The first statement we said to each other was, how did he get through the system?" Franz said. "How do they know somebody's background, especially that young? It is disturbing, to say the least."
Sheriff Keith Van Cleve said he would meet with state Attorney General J.B. Van Hollen on Monday morning to discuss the case.
Crandon Police Chief John Dennee said it would be handled by the state Department of Criminal Investigation because the suspect was a deputy and officer.
Peterson was killed Sunday afternoon, eight miles north of Crandon in the rural town of Argonne, Dennee said.
Crandon mayor Gary Bradley said Sunday that a sniper killed the suspect, but Van Cleve would not confirm that officers shot him.
Bradley expressed anger and frustration at the state's handling of the case, complaining about a lack of information for officials and families and the length of time being taken in removing the bodies from the home, reports the Sentinel.
"Man, they paralyzed this town," he said.
The gunman's motive was unclear, but the mother of a 14-year-old victim, Lindsey Stahl, said the suspect may have been a jealous boyfriend.
"I'm waiting for somebody to wake me up right now. This is a bad, bad dream," said Jenny Stahl. "All I heard it was a jealous boyfriend and he went berserk. He took them all out."
Dennee declined comment on whether Peterson had a romantic relationship with any of the victims.
The white, two-story duplex was about a block from downtown Crandon, a town of about 2,000 located 225 miles north of Milwaukee in an area known for logging and outdoor activities. The victims had gathered for what Dennee described as "a pizza and movie party."
Three of the victims were Crandon High School students, said school Superintendent Richard Peters, and the other three had graduated within the past three years.
"There is probably nobody in Crandon who is not affected by this," Peters said, adding that students "are going to wake up in shock and disbelief and a lot of pain."
Peters did not know whether Peterson had graduated from the 300-student school. But Crandon resident Karly Johnson, 16, said that she knew the gunman and that he had helped her in a tech education class.
"He graduated with my brother," she said. "He was nice. He was an average guy. Normal. You wouldn't think he could do that."
The Crandon School District called off classes Monday.
One victim, 20-year-old Bradley Schultz, was a third-year student at the University of Wisconsin-Milwaukee who was home to visit his friends, said his aunt, Sharon Pisarek.
"We still don't have many details, but from what they've told us, there was a girl next to him and he was covering her, protecting her," she said, sobbing. "He was loved by everybody. He was everybody's son. Senseless."
David Franz's wife, Marci, said she was awakened by the gunshots.
"I heard probably five or six shots, a short pause and then five or six more," she said. "I wasn't sure if it was gunfire initially. I thought some kids were messing around and hitting a nearby metal building."
Then she heard eight louder shots and tires squealing, she said.
"I was just about to get up and call it in, and I heard sirens," she said. "There's never been a tragedy like this here. There's been individual incidents, but nothing of this magnitude."