The gunman shot three people on the second floor of an Oregon mall, then ran.
Shoppers hid behind racks as the man, wearing a vest and a mask, ran down a corridor and to a back hallway that led downstairs.
By then, he likely heard the sirens as dozens of police vehicles converged on the Clackamas Town Center in Happy Valley.
He then took his own life.
As details of Tuesday's mall shooting emerged, the question of motive remained unanswered.
At a news conference on Wednesday, investigators identified Jacob Tyler Roberts, 22, as the lone suspect in the shootings -- two of which proved fatal -- at the suburban Portland mall.
Sheriff's investigators said there was no apparent connection between Roberts and any of his victims.
The fatalities were identified as Cindy Ann Yuille, 54, and Steven Mathew Forsyth, 45.
The incident unfolded during the afternoon as the gunman moved quickly through the Macy's at the mall and toward the food court on the second floor, where he opened fire, the sheriff's office said.
The arrival of police within a minute of receiving the first calls of the shooting may have influenced the gunman's ultimate course of action, Sheriff Craig Roberts said.
The gunman was wearing a load-bearing vest -- a military-style garment designed to carry heavy equipment, which some witnesses confused with a bulletproof vest -- and was carrying a semi-automatic rifle.
At one point, the rifle jammed but started working again, the sheriff said.
Kristina Shevchenko, 15, was taken to the Oregon Health & Science University Hospital, where she was in serious, stable condition on Wednesday with a bullet wound to her chest, her surgeon told reporters.
Shevchenko, who suffered a collapsed right lung and injuries to her liver, underwent an hourlong operation Tuesday night to remove bullet fragments and was in the hospital's intensive care unit, Dr. Laszlo Kiraly said.
He said he was optimistic that she would recover fully but added that she was in shock and at risk for complications, including infection. "She's a very brave young woman," he said, adding that she was awake and talking to her family.
Further surgeries will focus on closing her wounds "and getting her to heal properly," he said. "I would anticipate that she would be with us for about a week or so."
Friends of the gunman said they were shocked when they heard who the shooter was.
"This is more than a tragic event, and my heart goes out to everyone affected by it, but I also feel as if the side of Jake that I remember needs to be shared," said Erin Philpot, a friend of the shooter, in an e-mail to CNN.
"He was a very loved individual that always seemed as if he had great intentions and a heart of gold. I know that must be hard for people to see/think/understand, but It's the truth. I don't know what caused him to do such a horrendous, terrible thing, but I do hope that somehow answers will be available to us all."
Philpot, who attended middle school and high school with Roberts, said he had recently broken up with a girlfriend and was considering moving to Hawaii. "I just had drinks with him in November, and he had just moved into a new place and was totally stoked about it," she said. "I can't believe he did this."
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.
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.
"My 5-year-old was covering her ears and crying. I was frantic to find a place to run, and I looked back -- my son in my stroller and glass is falling over us," she said. "The shots were getting closer."
She added, "I felt like sitting ducks where we were."
During a brief halt in the gunfire, a man helped hustle the family members into a cosmetics store, where they hid for an hour "waiting to see if we were going to be shot or not."
As word spread that the shooter was moving from store to store, customers at Sears burst into tears, Christina Fisher told KOIN.
"We were told to stand in a group by the top of the escalators and stay away from the windows out of the aisle. ... We stood there for probably a good 20 minutes," she said. "All of the sudden, somebody came through with a radio, yelling, 'Get down!' "
As the mayhem unfolded, some customers watched television reports about the shooting from inside the Sears entertainment center, Tylor Pedersen told affiliate KGW.
Antonio Charro spotted a wounded woman near a cell phone store and tried to help.
"She had apparently been shot in the chest, and I couldn't get her turned over to help her," said Charro, who had been shopping at the mall with his daughters. "There was no one around. She wasn't breathing."
Clackamas County Sheriff Craig Roberts said about 10,000 people were in the mall at the time.
No law enforcement officers fired any shots when they arrived, sheriff's Sgt. Adam Phillips said.
The 1.4 million-square-foot mall remained closed Wednesday as investigators looked for clues about the attack.
Rowland said she's grateful she got distracted while shopping and didn't venture farther into the mall.
"I think if I hadn't stopped to smell that perfume, that maybe me and my baby wouldn't be here today."
CNN's Meridith Edwards, Catherine E. Shoichet, Cristy Lenz, Chandler Friedman, Michael Martinez, Tom Watkins and AnneClaire Stapleton contributed to this report.