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The afternoon shooting at the crowded shopping centre in the Portland suburb of Happy Valley touched off a panic inside the mall, with shoppers streaming out as police and fire crews arrived on the scene.
"I can confirm that we believe at this point that there was one and only one shooter involved and that that shooter is deceased," Clackamas County Sheriff's spokesman Lieutenant James Rhodes told a press briefing.
"In addition to that, we have at least one patient who was taken from the mall with a traumatic injury and at least two that were deceased in addition to the shooter," he added.
Rhodes did not say how the suspect was killed.
Police said they were working to evacuate terrified shoppers, who were reported by local media and via Twitter to have hidden in the back rooms of shops as gunshots rang out in the mall.
The Oregonian newspaper reported on its website that the gunman had used a semi-automatic rifle and may have also been wearing body armor, but police did not immediately confirm that.
The paper said the shooting occurred near the mall's food court.
"All of a sudden I heard something similar to a .22 popping, probably eight times, and people dropping everywhere, people screaming," witness Christina Fisher said in an interview aired on CNN. "It was chaos."
The identities of the dead and wounded were not immediately released. The gunman was also not identified and authorities did not release the type of weapon or weapons used.
Oregonian sports reporter John Canzano, who said he was at the mall when the shooting began, tweeted that "panicked people are running from the mall" and later urged his followers simply to "pray."
Police and SWAT teams established a perimeter around the scene and worked to evacuate the mall as they searched for the shooting suspect. The Clackamas County Sheriff's office urged members of the public to avoid the area.
A spokeswoman for the mall said she had no immediate information and directed calls to law enforcement authorities.