Clackamas County Sheriff Craig Roberts told the Associated Press that Roberts used a stolen rifle from someone he knew Tuesday night during the shooting.
Authorities have not yet discovered a motive for the shooting which left two dead and another person injured. The gunman was found dead in an apparent suicide.
Roberts reportedly did not have a criminal record before the incident aside from two speeding tickets this year, the Oregonian reported. The newspaper also reported that Roberts and a woman named Hannah Shoemake were evicted from their apartment last year.
The Oregonian found Roberts' mother at her Portland home, but she declined to comment, other than to say, "I loved my son very much."
Roberts open fired at the Clackamas Town Center mall in suburban Portland, Ore. Tuesday around 3:30 p.m. wearing a white hockey mask and black clothing. The shooting victims were identified as Cindy Ann Yuille, 54, and Steven Mathew Forsyth, 45. Kristina Shevchenko,15, was the injured victim taken to a hostpial for surgery after a bullet bruised her lung but avoided major organs.
Witnesses said Roberts stormed the mall through a Macy's and open fired in the mall food court using the rifle. Shoppers at the mall that day, which amounted to more than 10,000, took cover in storerooms and were led out by police who arrived on the scene within minutes.
Police said it appeared that Roberts was looking to kill "as many people as possible" who were strangers to him.
"We have been able to identify the shooter over this last night," Sheriff Roberts said. "I believe, at least from the information that's been provided to me at this point in time, it really was a killing of total strangers. To my knowledge at this point in time he was really trying, I think, to kill as many people as possible."
Police said Roberts had armed himself with an AR-15 semiautomatic rifle and was in the possession of several fully loaded magazines. The gun used by Roberts at one point jammed, halting his shooting rampage, but the 22-year-old unjammed the gun and began shooting again as many as 60 times. Police said they found Roberts dead from an apparent suicide, according to multiple reports.
Authorities said extra details surrounding the case would not yet be announced due to an ongoing criminal investigation.