Jordan Salazar, who attended Milwaukie High School in the Portland area with Roberts, said in an e-mail, "He was a really nice guy, and it's completely shocking he did this."
She added, "He was a popular guy in high school and had a lot of friends," she said. "I was pretty much in shock with all of this."
Roberts attended Milwaukie High School for three years but did not graduate, said Joe Krumm, the director of community relations for North Clackamas County School District. "Average talent, average grades," Krumm said about Roberts, who was in a mainstream program and had no record of discipline issues or extracurricular activities.
Roberts then transferred to another district and graduated in 2008 from Oregon City High School, the school said in a statement on its website that also talked about the killings.
"This news is very shocking to those who knew Jacob while at OCHS," the statement said. "He was known as a soft-spoken and polite young man who was often eager to be helpful. The motive for such a horrific act is likely to remain a mystery to us all."
The families of the victims who died -- Yuille and Forsyth -- provided brief comments through authorities but asked for privacy as they mourn.
Yuille was remembered as "everybody's friend" who put others first.
Forsyth had two children, a great sense of humor and a zest for life, his family said.
During the shooting, mall patrons said they had no clue where the gunman would fire next.
Entire swaths of Clackamas Town Center became silent except for the rifle blasts and screams. Even the mall's Santa dropped to the ground.
"I thought I was going to die," mall employee David Moran said. "The gunshots were so loud, it was very scary. ... Kids were crying. Parents were crying, too."
Kira Rowland was holding her 6-month-old baby in Macy's when the shots rang out.
"I threw my baby into the stroller and just started running, because everybody was screaming and everybody just started to run," she said.
The gunman wore a hockey mask and jogged through Macy's wielding a rifle, a woman told CNN affiliate KOIN.
As some panicked customers bolted for the exits, others ducked under store counters or hid behind clothing racks.
"I am the shooter," he said as he ran through Macy's, said witness and store employee Austin Patty.
As the shooting started, Patty ran from the store, warning all in earshot that a shooter was loose and ordering them out of the store.
The sheriff's office said a rifle and a mask were recovered from the scene.
Investigators were reviewing surveillance footage to get a better picture of what happened.
Erin Quackenbush-Baker was with her grandmother and three children at a kiosk in the middle of the mall when she heard the shots.