Prosecutor Denis Tracy announced Friday that he did not feel he could prove beyond a reasonable doubt who committed the assault.
The incident began when David Warner was involved in a fight last March 30 outside a Pullman bar. Police have said Warner was trying to break up the fight when he was knocked to the ground and suffered a serious head injury.
Five people were ultimately arrested in the case.
Warner, who taught in WSU's Department of Critical Culture, Gender and Race Studies, has had to undergo extensive speech and physical therapy.