times. the motive, prosecutors say, is still unclear. let s hope that it wasn t because of a uniform. let s hope it wasn t race related. reporter: now cnn has learned the suspect was previously ruled mentally incompetent back in 2012. miles was charged with aggravated assault against a homeless man. miles, also homeless at the time, was sent to a mental institution for six months. then ruled competent, but the case never went to trial. even given the suspect s mental health history, one texas sheriff suggests the killing could have some relation to the black lives matter movement and ongoing uprising against police brutality. it isn t a far stretch to believe that that kind of rhetoric could influence someone. reporter: leaders of the movement say calls for police reform cannot be blamed for this senseless murder. our mission is to end violence against black people