What evidence is discovered that affords Jamal and his father their freedom in the novel, This Is My America?

When Chris takes Richard's gun for fun and lets one of his friends borrow it, the gun is stolen by Angela who attempts to use it to infiltrate their hate group. Richard Brighton soon discovers her plan and kills her. The discovery of the gun, however, ultimately allows the police to pin both crimes on Richard Brighton, setting Jamal and his father free at last.