John Zawahri, 23, the shooter in Friday's rampage, may have assembled the rifle himself to circumvent the state's restrictions on the firearm, sources told the Los Angeles Times.
Certain types of AR-15 rifles are banned in California, but it is legal to buy parts to assemble and customize the guns.
Authorities, who aren't sure if Zawahri obtained the gun whole or constructed it himself, are attempting to track down the origin of the parts.
California law outlaws AR-15s that have a magazine capacity of more than 10 rounds, but Zawahri had 40 magazines capable of holding 30 rounds each, sources told the Times.
Zawahri fatally shot six people Friday during a rampage that started at his father's home and ended at Santa Monica College where police shot and killed him.
Authorities said Wednesday Zawahri fired about 100 shots and had access to more than 1,300 rounds of ammunition.
AR-15s also were used in the Aurora, Colo. movie theater shooting and the Sandy Hook massacre.