But not every single crime can be solved with forensic evidence.
People say hope springs eternal. Polly Sullivan lived those words.
"She was always what I would call a do-gooder," Linda Gruno, Sullivan's sister said. "She had this really naïve attitude that all people were basically good, if you help them, if you gave them the chance they would prove that to you."
The 44-year-old wanted to give others hope.
"It was supposed to be just a home for single Indians coming to integrate into the city, but she took a lot of people from the Department of Corrections, she helped people with their parole plans, she went down to Canon City and picked them up, she gave people clothes, bus fair money," Gruno said.
Crooked Tree was a housing development at Lowry which Sullivan helped establish and managed for about a year.
"I used to tell her that she needed to get a gun and take a class and learn how to use it, she goes I'm not afraid of anybody, I'll just call the police if anything happens," Gruno said.
The day after Christmas in 1998 someone else had to call police to Sullivan's home.
She was stabbed 14 times. The murder weapon was never found. Apartment resident Willie Safford was arrested and charged in connection with the killing. The charges were dropped a month later.
Gruno puts it on Denver Police.
"The whole investigation was kind of messed up from the start," she said. "But this case is solvable, every expert has looked at it has said this case is solvable. And why they're taking so long to do something about it."
Matt Murray was a detective when Sullivan was murdered and took on the case.
"In the end we did not have either the evidence or the testimony to continue to hold and try Willie Safford," he said. "What really happened in this case was all of what looked to be fantastic evidence to help us prove our case just kind of started to fall apart. We'd hoped blood in one place would tie one person and blood in another place would, but none of it did. And that's one of the reasons forensics is not the answer to everything."
Police can't tell 9NEWS what evidence they have, since this is still an open case.
But say they've tested and will continue testing the evidence.
"The evidence actually rarely solves the case," Murray said. "The evidence leads the investigator to how to solve the case. We continually apply new science, new technology in an effort to further the case forward through both evidence and any type of information we can obtain from other people. We don't give up, we don't quit."
Hope for the Gruno family is a delicate word.
"I don't have very much hope that they will do anything within the next five years," Gruno said.
Because no matter how Linda Gruno is "jaded" in her words about what Denver Police can do for her. She's not giving up, which means just like her sister Polly Sullivan, who always believed in people, Linda has hope.
"We're not going to stop, we're gonna keep going. We'll never stop they will always be bothered by us. Because I believe it can be solved. I also believe police are there to serve and protect, not just arrest people."
Denver officially formed its Cold Case Unit in 2004. Denver District Attorney Mitch Morrissey told 9NEWS Crime and Justice Reporter Anastasiya Bolton the unit has solved more than 80 cases in a decade.
More specifically, 87 cases broken down to 70 sexual assaults, 14 homicides, 1 burglary, 1 first-degree kidnapping and 1 attempted homicide.
He said U.K. comes the closest to the number of cold cases solved with 40 resolved.
Between 1970 and 2010 Denver has a total of 678 homicide cases that are open or unsolved.
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