* Semiautomatic rifle, which jammed, stolen from acquaintance of suspect
* Gunman, 22, who wore hockey mask, shot himself to death
* Surviving victim, 15-year-old girl, in serious condition (Updates with information from news conference)
By Teresa Carson
CLACKAMAS, Ore., Dec 12 A masked gunman who opened fire in the food court of a crowded Oregon shopping mall, killing two people and wounding a third before taking his own life, appears to have acted alone in a blind rampage with no known motive, police said on Wednesday.
Investigators identified the man behind Tuesday afternoon's shooting as Jacob Tyler Roberts, 22, who they said had no significant criminal history and apparently acted without warning.
The weapon he used was described as an AR-15 semiautomatic rifle, which the Clackamas County Sheriff's Office said was stolen the day before the shooting from an unidentified acquaintance of the killer.
Roberts also was 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 at the center of the mall to start shooting, authorities said. He was wearing a hockey-style mask at the time.
Two people were killed on the spot - 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, according to Sheriff Craig Roberts.
Kristina Shevchenko, 15, was badly wounded but managed to stagger away from the food court to the lower level of the mall, where she was found. She was hospitalized in serious condition on Wednesday.
But carnage from the shooting likely was limited by the fact that the gun jammed, although the suspect managed to get the weapon working again before he moved downstairs and shot himself, the sheriff told a news conference.
Sheriff Roberts also said the casualty count was curtailed by the fact that the estimated 10,000 shoppers in the mall "kept a level head" for the most part in swiftly leaving the building, and helping each other in the process.
He credited police with rapidly swarming the mall and merchants with following emergency lockdown procedures that allowed many shoppers to quickly find shelter.
'WE DO NOT UNDERSTAND THE MOTIVE'
Authorities said they remained baffled about what led the suspect to open fire in the mall, the latest in a spate of U.S. gun violence incidents this year, most notably the killing of 12 people and wounding of 58 others at a midnight screening of the Batman film "The Dark Knight Rises" in Colorado in July.
"At this time, we do not understand the motive of this attack except to say there is no apparent connection between the suspect and his victims," the sheriff said.
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 sheriff's officials said they had little if any immediate information about the gunman's personal background, employment history or education.
Phillips said Roberts was known to have once been a victim of a crime, but he did not elaborate, and it was not clear whether investigators believed that may have had any bearing on the shooting.
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.
High school student Hannah Baggs, 14, told the Oregonian newspaper that she got a close look at the gunman before he entered the mall and opened fire.
"He was, like, 10 feet (3 meters) away from us, wearing a white mask and carrying something heavy with both hands," Baggs said in a report on the newspaper's website. "He went running into the store." (Additional reporting by Chris Francescani in New York; Writing by Steve Gorman; Editing by Cynthia Johnston and Bill Trott)