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The US Bureau of Alcohol, Tobacco, Firearms, and Explosives provided the information late Thursday in an incident report obtained by the Associated Press.
The guns included a 9 mm Glock pistol and .40-caliber Smith and Wesson, both traced to the suspect.
He also had a .40-caliber Taurus pistol traced to someone in Portland, Oregon, and a .556-caliber Del-Ton.
The shooting Thursday at Umpqua Community College also wounded seven people. Officials identified the killer as Chris Harper Mercer. He died after a shootout with police.
He isn't believed to have a criminal history. Investigators believe he may have been a student there because a receipt found at the scene showed he purchased textbooks from the campus bookstore two days before the shooting.