CHATTANOOGA, TN (WRCB) -- Police are now calling a Chattanooga woman's death a homicide. Deborah Underhill was last seen alive at the AIM Center in downtown Chattanooga on March 28. The next day, two fishermen found her body on the south shore of the Tennessee River just off Patten Chapel Road.

The medical examiner's report gives new insight into how Underhill died, listing strangulation and blunt force trauma to her chest and abdomen as the cause of death.

Deborah's sister says she was a strong person and not the least bit surprised she put up a good fight.

"She put up the fight for her life and she lost," says Karen Searle, holding back tears.

Searle clings to the autopsy report that reveals how her sister, 57-year-old Deborah Underhill, died.

"This person committed a horrific crime and you've got to pay for that," says Searle.

The report shows Underhill was beaten and bruised over much of her body and ultimately strangled to death.

"They're most definitely sick. And to take advantage of someone that, you could look at her and tell she was mentally unstable."

Searle says her sister struggled with mental problems but brought joy to anyone she met.

"Debbie was so loving and caring and such a joy to be around. She was just the life of the party, she was just ready for everybody to be happy," says Searle.

She can not understand who would want to hurt her sister.

Detectives are trying to piece together what happened from the time Deborah left the AIM Center, to more than 24 hours later, when her body was found on the banks of the Tennessee River, more than five miles away.

"If this was somebody she loved, not just to mention me, she loved a lot of people, she would have already been her own detective. And I'm trying to give the detective time," says Searle.

In the meantime, she is determined to keep her sister's name out there to find answers.

"I know God will take care of this. He'll take care of this. I pray that God will put such a conscience on this person, it'll come out."

We spoke with the president of the AIM Center off camera. He said he could not talk about the case due to HIPAA violations, but expressed his condolences to Deborah's family.

Police confirm they have pulled surveillance video from the center and the surrounding areas to look for clues as to where Deborah may have gone.

They tell us Deborah Underhill sometimes used the name Deborah Faye Swafford.

Investigators say she frequented the downtown area, particularly around the public library.

Police have received few leads in this case.

If you know anything, or think you may have seen Deborah Underhill around the time of her death, call Chattanooga Police at (423) 698-2525.