How do i fix this problem?

im an e5 in the 82nd. Roughly 3-4 weeks ago one of my former soldiers was pulled into the office by an e5 and e6 from a different platoon by themselves. they they proceeded to punch him in the face and gut, then threatened to slit his throat if he told anyone. he then came running out, read marked faced and traumatized, and told me the story. we then called the MPs and a questioning was done by them and my first seargent to the victom and the two worthless NCOs who did the ganging. word was said that next week CID was coming down to further the investigation but never came. its been 3-4 weeks since the incident and seems to be that its been swept under the rug due to favortism by the first seargent and seargent major liking the two dirt bags. they are still the same rank, same position, and none punishment received, while the victom states that he doesnt sleep any more for fear that they will kill him for telling. anyone have any advise as to how i can fix this? as an NCO this absolutely fires me up as it should any true NCO in the army.