One witness at the Clackamas Town Center told television station KATU at least two people were shot, and he saw sheets put over them.
Another witness told the station she saw people screaming and parents grabbing their children as they fled the mall.
That witness said the gunman had a rifle and was wearing a camouflage outfit and a mask.
Another witness said ambulances were arriving at the scene.
Entrances to the mall have been blocked off.
No further details were immediately available from the Clackamas County sheriff's office.