Circuit Judge Gary Oxenhandler ruled Tuesday that prosecutors failed to turn over key evidence to Mark Woodworth's lawyers. Woodworth was 16 when his neighbor, Cathy Robertson, was shot to death while she slept in her northwest Missouri home.
Her husband, the business partner of Woodworth's father, survived the shooting. Woodworth is serving a life sentence for murder.
The state Supreme Court appointed the Columbia judge as a special master and will decide whether to release Woodworth from prison or order he stand trial for a third time.
Oxenhandler's ruling follows a weeklong hearing in June 2011.