An AR-15 rifle, the kind of weapon used by an off-duty sheriff's deputy after he burst into a home early Sunday in Crandon, Wis., and killed six people, is shown.

Crandon, Wis. ? A young sheriff’s deputy who opened fire on a pizza party and killed six people reportedly flew into a rage when he was rebuffed by his old girlfriend, and others at the gathering called him a “worthless pig.”

A longtime friend told the Milwaukee Journal Sentinel on Monday 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 911.

Peterson told Kegley that he had gone to his ex-girlfriend’s house early Sunday morning in hopes of patching up the relationship after a recent breakup. But, he said, Peterson lost control when the meeting ended in an argument and other people started ridiculing him as a “worthless pig.”

Kegley declined to comment when reached by The Associated Press.

Police, who declined to provide details of the argument, said 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,” Crandon Police Chief John Dennee said at a news conference Monday.

Peterson, a deputy and part-time police officer, died after exchanging gunfire with law enforcement officers. Whether Peterson was shot by police or took his own life was unclear.

The rampage raised questions in the remote northern Wisconsin community of 2,000 about how Peterson could have met requirements to become a law enforcement officer, especially after police acknowledged Monday that Peterson received no psychological screening before he was hired.

Some questioned the wisdom of hiring someone so young.

“No person that I’ve ever known at 20 years old was responsible enough to be a police officer,” said Steve Bocek, of Oak Creek, whose nephew Bradley Schultz was killed. “It’s unbelievable. You don’t have the mind to be a police officer. It takes a lot.”

But Crandon city attorney Lindsay Erickson said age doesn’t matter as long as officers do their jobs well. Peterson testified for her in several cases. He wrote good reports and was “true to his job,” she said.

“From what I saw of him, I didn’t see any warning signs or red flags,” Erickson said.

Peterson was hired as full-time deputy sheriff on Sept. 11, 2006, at the age of 19, according to personnel records released by the Forest County clerk. His yearlong probation ended last month.