* Semiautomatic rifle stolen from acquaintance
* Gunman, who wore hockey mask, committed suicide -police
* Surviving victim, 15-year-old girl, in serious condition (Adds statement from gunman's mother, fifth paragraph)
By Teresa Carson
CLACKAMAS, Ore., Dec 12 (Reuters) - A masked gunman who opened fire in a crowded Oregon mall in the middle of the holiday shopping season, killing two people and wounding a third before taking his own life, appears to have acted alone in a random rampage, police said on Wednesday.
Investigators identified the man behind Tuesday's shooting as Jacob Tyler Roberts, 22, who they said had no significant criminal history and acted without any obvious warning, though his Facebook page indicated an affinity for guns and alcohol.
Roberts' page on the social media site lists "shooting" as one of his interests but offers no clues that he may have been preparing for a shooting spree.
"I'm the conductor of my choo choo train," he wrote on the site. "I may be young but I have lived one crazy life so far. My friends are my family and I don't think that will ever change."
Portland-area television station KGW published on its website a note of apology it attributed to Roberts' mother, Tami Roberts, saying she had "no understanding or explanation" for his behavior, adding that it was "so out of his character."
The weapon used in the shooting was an AR-15 semiautomatic rifle, which the Clackamas County Sheriff's Office said was stolen the day before from an acquaintance of the killer.
Roberts was also believed to be carrying several fully loaded ammunition magazines when he walked into the Clackamas Town Center in the Portland suburb of Happy Valley and moved swiftly to a food court to start shooting, authorities said.
Two people were killed - Steven Mathew Forsyth, 45, of suburban West Linn, a father of two who owned a business in the mall, and Cindy Ann Yuille, 54, of Portland.
Kristina Shevchenko, 15, was wounded but managed to stagger away from the food court to the lower level of the mall, where she was found. She was in serious condition in an Oregon hospital.
But the carnage likely was limited because the gun jammed, although the suspect managed to get it working again before he moved downstairs and shot himself, Sheriff Craig Roberts told a news conference.
He also said the casualty count was curtailed because the estimated 10,000 shoppers in the mall "kept a level head" for the most part in swiftly leaving the building.
Authorities said they remained baffled about what prompted the violence, the latest in a spate of U.S. gun rampages this year including a deadly July shooting at a midnight screening of a Batman film in Colorado that killed 12 people.
Sergeant Adam Phillips, a spokesman for the sheriff's office, said investigators were not aware of any signs that Roberts was preparing to act as he did.
"Every indicator is that he acted solely alone in carrying out this heinous and tragic crime," he said.
Detectives contacted several members of the suspected gunman's family as they began their investigation, but authorities gave little information about Roberts' personal background, employment history or education.
A photograph of Roberts displayed by police showed a young man with longish, straight black hair, a light mustache and goatee, and ring-sized discs in each of his pierced earlobes.
Roberts' Facebook page offered few additional insights into the man suspected in the murderous rampage.
He described himself in a brief biography as "a bit of an adrenaline junkie," and as "a pretty funny person that takes sarcasm to the max." Professing an independent streak, Roberts said: "I'm the kind of person that is going to do what I want."
He starts out jokingly introducing himself as "Jake and I'm an alcoholic," then goes on to write that in addition to "hanging out with my friends" he likes to "maybe get a little drunk every now and then."
His profile includes a snapshot of a person firing a pistol at two outdoor targets, though the face is not visible because the photo was taken from over the subject's shoulder.
In addition to "shooting," he lists river rafting and camping among his interests. He also counts the cable television dramas "Dexter," about a serial killer, and "Breaking Bad," about a high school teacher who becomes a methamphetamine kingpin, among his favorites, along with "South Park." (Additional reporting by Chris Francescani in New York and Steve Gorman in Los Angeles; Writing by Steve Gorman; Editing by Cynthia Johnston and Eric Beech)