The Milwaukee Journal Sentinel reports the district attorney's office reviewed the police investigation, interviewed witnesses and viewed cellphone video before concluding that Jeremy Rossetto shot in self defense.
Police said 17-year-old Anmarie Miller and 19-year-old James Bell Jr., both of Milwaukee, were killed in an apartment building stairwell March 12.
Prosecutors are still reviewing the case against a 20-year-old who is accused of punching Rossetto.
Police said the three younger people got into an argument with the 39-year-old Rossetto. Police say one of the three was holding him while another beat him with a souvenir baseball bat before Rossetto drew a gun and fired.