From reading other news sources, he sold the guns to an FBI informant after the shooting. The FBI did ballistics testing after they bought them and found that they were the guns from the high school shooting. It’s not clear how he got them, but presumably he got them illegally. There was never an announcement about recovering the guns, and due to the nature of the crime the city has been vocal about every bit of positive news in the case. That makes me think he got them on the street rather than stealing them from an evidence locker as some have speculated.