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.

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