Christian Philip Oberender killed his mother with a shotgun in 1995, when he was 14.
He was committed to a state hospital after being deemed mentally ill and a possible threat.
He's since been released, and in May, he got a legal gun permit in Minnesota and amassed an arsenal of 13 guns -- including an AK-47 and a Tommy gun. He began to post photographs of the guns on Facebook along with notes sympathizing with the gunmen at Columbine High School and Sandy Hook Elementary School, officials said.
Oberender, 32, was arrested Jan. 2 by Carver County sheriff's deputies on suspicion of being a felon in possession of firearms, with bail set at $100,000.
Police found a note in his bedroom, according to KARE-TV: "I feel the good part of me fade away. I don't know how long I can hold it in for. I think about killing all the time. ... The monster want out. I know what happens when he comes out. He only been out one time and someone die."