A young sheriff’s deputy who opened fire at a pizza party and killed six people reportedly flew into a rage when he was rebuffed by his old girlfriend and mocked by party guests as a "worthless pig".

A longtime friend told the Milwaukee Journal Sentinel yesterday that 20-year-old Tyler Peterson came to his door in the hours after the rampage and calmly explained what he had done.

“He wasn’t running around crazy or anything. He was very, very sorry for what he did,” Mike Kegley told the newspaper, adding that he gave Peterson coffee and food and later called the authorities.

Peterson told Mr Kegley that he had gone to the house of Jordanne Murphy, his former girlfriend, early on Sunday morning in hopes of patching up the relationship after a recent break-up, but that he had lost control when the meeting ended in an argument and other people started ridiculing him.

Police did not confirm details of the argument, but said that Peterson stormed out, retrieved an AR-15 rifle from his car outside and burst back into the house firing 30 shots that killed all but one of the people at the party.

“We had no idea, obviously, that anything like this would ever occur,” said John Dennee, the police chief of the small town of Crandon, at a news conference last night.

After the attack, Peterson identified himself as the gunman in phone conversations with the police chief and others. The rifle used in the shootings is the type used by the sheriff’s department.

It is understood that he fled to a neighbouring town where he took a family hostage.

Peterson, who was also a part-time police officer with Crandon Police Department, died some hours later after exchanging gunfire with a team of armed police. It was not clear whether he was shot by the Swat team or took his own life.

Some details of the killings remained sketchy. John “JB” Van Hollen, Wisconsin’s attorney general, told the press conference: “Violence so terrible and tragic will never make sense. We’re seeking answers too.”

Questions have been raised in the remote northern Wisconsin community about how Peterson came to be hired as a law enforcement officer, especially after it emerged that he had received no psychological screening.

He was appointed a full-time deputy sheriff on September 11, 2006, at the age of 19, according to personnel records released by the Forest County clerk, and only completed his year-long probation period last month.

Mr Dennee told reporters at the news conference that Peterson and the young woman had been in a relationship for a few years. “They had broken up and gone back and forth,” Dennee said.

Three of his victims were still at Crandon High School, while the others were, like Peterson, recent graduates. They were at the house to share pizza and watch movies during the school’s reunion weekend.

The other victims were identified as teenagers Katrina McCorkle, Leanna Thomas and Lindsey Stahl, and two of Peterson's childhood friends, Aaron Smith and Bradley Schultz. Post-mortem examinations were being carried out yesterday, but the results have yet to be issued.

Schultz, 20, was a third-year student of criminal justice at the University of Wisconsin-Milwaukee who hoped to be a homicide detective. He was home visiting friends and appeared to have died trying to protect one.

“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,” said an aunt, Sharon Pisarek, as she sobbed. “He was loved by everybody. He was everybody’s son. Senseless.”

The lone survivor, Charlie Neitzel, 21, from the neighbouring town of Pickerel, was showing signs of improvement last night, and was said to be in serious condition in hospital.

Pastor Bill Farr read a statement from Peterson’s family in which they expressed their shock and sorrow at what he had done.

“Our hearts go out to the victims, their families and their friends. We are grieving for your losses. We feel a tremendous amount of guilt and shame for the acts Tyler committed,” it said.

“We may never receive the answers we all seek. Like those close to Tyler we are in shock and disbelief that he would do such terrible things. This was not the Tyler we knew and loved.”

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