MARSHFIELD, Wis. -- The mother of a slain University of Wisconsin-Madison student is speaking publicly for the first time, hoping to draw public help in finding her daughter's killer.

Jean Zimmermann of Marshfield told the Wisconsin State Journal that healing can't start until there's justice for Brittany Zimmermann. She's the 21-year-old found dead in her apartment in April after placing a fruitless 911 call.

Jean Zimmermann said she doesn't understand how no one on the busy streets around the apartment saw the killer.

She said she's also haunted by the memory that Brittany tried calling her that day but she missed the call.

Madison police said that they have a hypothesis in the case but are withholding details to avoid compromising the investigation.