The FBI had him under investigation and he should have been a known purchaser of a assault type rifle.
He probably didn't buy it. He may have stolen it, or had someone else buy it for him. But, an SKS is not an "assault type weapon. It simply a semi-automatic firearm, firing a cartridge with ballistics similar to a 30-30 used for deer hunting for probably over a century. And only marginally higher rate of fire.