Peake was arrested after police followed him to his storage unit and later searched it. They found three guns: Getgen's, Peake's, and a gun that had been reported stolen from the same range earlier.
He told police he didn't kill Getgen; according to Peake, he was already dead when Peake found him. Then he stole his gun.
From the police affidavit:
Peake said he has been stealing guns for the purpose of aiding an organization that Peake refused to name. Peake said the organization is collecting guns for the purpose of overthrowing the federal government (of the United States of America). Peake said he and Tuso together are members of this organization. Peake told your affiant that he would kill to defend his country and he was stealing weapons to defend his country.