PORTLAND, Ore. (AP) — Gunfire rang out in the mall food court, instantly transforming a casual afternoon of holiday shopping into a nightmare. The shooter, armed with a rifle, was dressed in dark clothing and wore a hockey-style face mask.

As panicked shoppers fled for cover, workers ushered some into hiding places within stores, or helped them to the exits. The first officers to arrive formed groups and rushed into the chaos, rather than waiting for the more heavily armed SWAT team.

"If we would have run out, we would have ran right into it," said Kaelynn Keelin, who saw a window get shot out and, along with other Made In Oregon co-workers, pulled customers into the store for shelter.

The quick mobilization of mall workers and police reflects the reality that, while mass shootings are rare, they have forced authorities to rehearse for such outbreaks of violence as if they are the norm.

"This could have been much, much worse," Clackamas County Sheriff Craig Roberts said.

Roughly 10,000 people were inside the Clackamas Town Center on Tuesday afternoon, when police say Jacob Tyler Roberts, 22, armed himself with an AR-15 semiautomatic rifle he stole from someone he knew, and went on a rampage that left two people dead.

The sheriff said the rifle jammed during the attack, but the shooter managed to get it working again. He later shot himself. The sheriff and Roberts are not related.

As authorities tried to determine a motive for a shooting they said had no specific targets, details emerged about Roberts from acquaintances and neighbors. They described him as relaxed, friendly and outgoing.

"Jake was never the violent type," Roberts' ex-girlfriend, Hannah Patricia Sansburn, told ABC News. "His main goal was to make you laugh, smile, make you feel comfortable. You can't reconcile the differences. I hate him for what he did, but I can't hate the person I knew because it was nothing like the person who would go into a mall and go on a rampage."

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Sansburn said Roberts had recently quit his job at a gyro shop in Portland and sold all of his belongings, telling her that he was moving to Hawaii. He was supposed to take a flight Saturday but told her he got drunk and missed it.

"And then this happens. ... It makes me think, was he even planning on going to Hawaii?" Sansburn said.

Sansburn did not return phone messages left by The Associated Press, and no one answered the door at her home Wednesday.

A former neighbor of Roberts said that he liked to play video games and never seemed troubled.

"He was like a rapper. He would rap all the time," said Samantha Bennett, who said she went to middle school with Roberts but wasn't close to him until he moved in with a girlfriend across the hall from her at an apartment complex in summer 2011.

His dining room was decorated like a jungle, Bennett said, with vines on the walls and a monkey. He once showed her a black handgun that she believed he purchased legally. He dropped out of sight earlier this year, and his phone was disconnected, she said.

On a Facebook page that a friend identified as Roberts', a few photos show Roberts hanging out with friends, while the cover photo is of a wall painted with the message "Follow Your Dreams" with the word "Canceled" stamped across the message.

In the public portion of his page, Roberts wrote, "I may be young but I have lived one crazy life so far."

"I'm the kind of person that is going to do what I want," he wrote. "There is no reason for another person to tell you what to do, I'm the conductor of my choo choo train."

He added he was "a bit of an adrenaline junkie" who is "just looking to meet new people and see the world."

Roberts rented a basement room in a modest, single-story Portland home and hadn't lived there long, said a neighbor, Bobbi Bates. She said she saw him leave at 1:30 p.m. Tuesday wearing a dark jacket and jeans, carrying a guitar case.

Roberts had several fully loaded magazines when he arrived at the mall, the sheriff said. He parked his 1996 green Volkswagen Jetta in front of the second-floor entrance to Macy's and walked through the store into the mall and began firing randomly in the food court.

He fatally shot Steven Mathew Forsyth, 45, and Cindy Ann Yuille, 54, the sheriff said. Kristina Shevchenko, 15, was wounded and in serious condition Wednesday, police said.

On average, there are about 20 mass murders every year, and the trend has been steady since the 1970s, said James Alan Fox, a professor of criminology at Northeastern University in Boston and author of five books on mass murder.

The typical mass murderer is a socially isolated white man in his 30s or 40s. He has no criminal record, but his life has been marked by failure and frustration, and he decides that his family, boss or co-workers must pay with their lives, Fox said.

"The rarest form of mass murder is the completely random shooting," Fox said. "Those perpetrators tend to be younger (in their 20s). They are more likely to have profound mental health issues, as opposed to the older guy who is quite sane, knows exactly what he is doing, and just decides that life is miserable."

The random shooter, Fox said, feels "the whole world is unfair, someone has to pay, and it doesn't matter who."

Amid Tuesday's gunfire, employees helped shoppers get into backrooms.

"Basically, in a situation like this it's either stay right where you're at and lock yourself down, or get to the nearest exit," said Dennis Curtis, the mall's senior general manager.

"We've done drills with the sheriff's office," including one earlier this year, he said.

The first 911 call came at 3:29 p.m. Tuesday, and officers arrived a minute later. Instead of waiting for SWAT teams, police immediately entered the crowded mall.

Police told people inside to put their hands in the air, to make sure an armed person was not among them. Police spent hours clearing the 1.4-million-square-foot mall, as some workers and shoppers continued to hide in fear.

Roberts fled along a mall corridor and into a back hallway, down stairs and into a corner where police found him dead from an apparent self-inflicted gunshot, authorities said.

Families of the victims released brief statements through the sheriff's office.

Relatives of Yuille described her as "everybody's friend" and a caring person. Forsyth was a loving husband, business owner and a youth sports coach, his family said.

As for Shevchenko, it was her second brush with death this year. In August, a man veered his car into the opposite lane and crashed head-on into a van she was in.

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Contributing to this report were Associated Press writers Jonathan J. Cooper, Nigel Duara, Anne M. Peterson, Tim Fought and Sarah Skidmore in Portland, Jeff Barnard in Grants Pass, Ore., Michelle Price in Phoenix, Pete Yost in Washington and Manuel Valdes in Seattle, and researcher Rhonda Shafner.