The Carson City Sheriff says Eduardo Sencion used a legal, Chinese-made version of an AK-47 assault rifle that was altered to work as an illegal automatic weapon.
Four people were killed and seven others were injured in the September 6 shooting at the IHOP restaurant.
Sencion took his own life at the scene.
Police say Sencion's mental health history has no bearing on the investigation because Nevada law allows the private sale of weapons without background checks.
