NEW YORK ( CNN ) -- Brittany Zimmerman , a 21-year-old college student who wanted to be a doctor , called 911 as she was being attacked by a stranger , police say . Brittany Zimmerman 's screams and struggle for her life were captured by a 911 tape . But the police did not come for 48 minutes . By that time , Zimmerman was dead . Her fiance found her body . Although the dispatcher claimed later to have heard nothing , the 911 tape captured screams , gasps and what sounds like a struggle , according to the court documents . Spring was in the air when college student Zimmerman returned April 2 from classes at the University of Wisconsin to the off-campus apartment she shared with her fiance , Jordan Gonnering . He was out when she arrived home . He discovered her body when he returned . Zimmerman had been stabbed multiple times in her chest , near her heart . She 'd also been beaten and strangled , according to warrants released recently . Watch an update on the case » . Zimmerman managed to call 911 at 12:20 p.m . The call was taken by the Dane County 911 center and an internal investigation revealed the dispatcher did not hear any sounds that would signal an emergency . Because of that , police were not sent to the apartment until 48 minutes after Zimmerman made the call . Her fiance was already there . Dane County has taken some harsh criticism from the public regarding the delay , and tough questions have been raised about whether a prompt response might have saved Zimmerman 's life . Police are still looking for her killer . `` We are working diligently on this case , have generated significant leads , and are making progress , '' said Joel De Spain of the Madison Police Department . The police said they believe Zimmerman was attacked by a stranger . Her apartment door showed signs of forced entry . After interviews with Zimmerman 's family , friends and acquaintances , investigators determined there was no personal motive for the attack . `` In fact , we have not been able to determine any motive yet in this case , '' De Spain said . He emphasized that police have no reason to believe Zimmerman was the victim of a serial killer . During the investigation , police have tracked leads pointing to vagrants in Zimmerman 's off-campus neighborhood . The vagrants often would knock on doors and beg for money . `` We are still investigating this avenue , but at this time we have not been able to develop any specific suspects , '' De Spain said . Zimmerman 's family and friends describe her as a loving , warm young woman , who had much to look forward to . She was engaged to the love of her life and had dreams of earning a medical degree , they said . She was idealistic , and her goal was to help people , not to earn a large salary , they said . Other details in the released warrants reveal that Zimmerman was murdered in her bedroom , that her cell phone was found in `` parts , '' and that her bloody slippers and bloody computer paper were recovered . The murder weapon is described as a knife , two to five inches long . Police are not saying whether they have recovered it . DNA was collected from Zimmerman 's body , as well as hair , blood samples , footprints and fingerprints . So far , no match has been made to a suspect . Zimmerman 's family is offering a $ 14,000 reward , and Crime Stoppers is offering $ 1,000 for tips leading to the arrest and/or conviction of anyone responsible for Zimmerman 's death . Please call the tip line at 608-266-6014 .
College student 's battle for her life caught on 911 tape . Police did not respond to her call for help for 48 minutes . Brittany Zimmerman 's fiance found her body . Police suspect she was attacked by a stranger and are eyeing vagrants .