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.
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.
- Photos: UCLA wades through damage caused by...
- Obama to GOP: 'Stop just hating all the time'
- Child sex trafficking bust reveals worries...
- Probe exposes flaws behind HealthCare.gov...
- GOP: Lois Lerner's IRS emails show bias...
- GOP-led House ready to OK lawsuit against Obama
- GOP-led House approves lawsuit against Obama
- Most Americans believe the U.S. should offer...
- Obama to GOP: 'Stop just hating all the... 45
- US Court: Virginia marriage is for all... 44
- Most Americans believe the U.S. should... 27
- GOP-led House ready to OK lawsuit... 23
- Obama maintains busy fundraising... 22
- GOP: Lois Lerner's IRS emails show bias... 21
- Fast food workers vow civil disobedience 18
- Democrats have million-dollar day on... 13