According to Bureau of Justice Statistics, in 1990, police made arrests in 72 percent of the homicide cases in the United States. In 2002, investigators solved 64 percent of homicides. Experts say uncooperative witnesses and the changing nature of homicides have contributed to the rise of unsolved slayings.
"Over the past 20 years, we've seen a decline in murders within families, among married couples. Those are typically a slam dunk for the police to solve," said James Alan Fox, professor of criminology at Northeastern University. "We've seen a growing number of youth homicides, drug-related homicides, gang homicides."
The DNA Revolution
Still, evolution of forensic technology — particularly DNA technology — has led to arrests in many cases where the trail had been considered hopelessly cold.
The 1993 strangling of Mia Zapata, the lead singer for up-and-coming alternative rock band The Gits, baffled Seattle police until last January, when authorities said DNA linked Florida fisherman Jesus Mezquia to the slaying.
Detectives from the King County, Wash., cold case squad had submitted the DNA profile of evidence from saliva found on Zapata to the National DNA Index System in November 2002. Mezquia had submitted a sample as part of a previous, unrelated arrest in Florida.
According to a criminal complaint filed against him, in July 1993 Mezquia was living within walking distance of the area where Zapata was killed. He has denied even knowing Zapata, much less killing her, but cannot explain to investigators how his DNA wound up on her body. Mezquia is currently awaiting trial.
DNA evidence led to the 2001 arrest of Gary Ridgway in the Green River slayings, which claimed the lives of 49 Seattle-area women, including several prostitutes, in the 1980s. In November, Ridgway confessed to killing 48 women and subsequently received multiple life sentences in prison in a plea deal to avoid execution.
"Without a doubt, DNA evidence has been the biggest advance in cold case investigations," said Jackson, of the Kansas City cold case squad. "And there are advances being made every week. It has enabled a piece of evidence that detectives were not able to use 20 years ago to become a crucial part of the investigation."
Hope Amid an ‘Emotional Roller Coaster’
Jackson said he has six detectives in his cold case squad and each juggles, on average, four investigations. All are seasoned homicide investigators: The least-experienced member of Jackson's squad was in the homicide unit for nine years.
He stressed that cold case investigations are not as glamorous — nor nearly as quick to resolve — as the ones viewers love to watch on television.
"Nothing happens overnight," Jackson said. "This is not what you see on CSI or Cold Case, where cases are solved in an hour. Real police work takes time."
Meanwhile, Kansas City's Sgt. Bernard maintains that he and his investigators are hopeful that they will — eventually — uncover Precious Doe's identity and find her killer.
Officials are not even sure of Precious Doe's precise age. They initially estimated that she was between 3 and 6 years old at the time of her death. An examination by a forensic dentist released in September found that she was 3 or 4 years old when she was killed.
In September, the Kansas City Police Department unveiled a new bust of the slain child created by the FACES Laboratory at Louisiana State University. The bust was constructed using the girl's skull as a model and is considered the most realistic portrait of the child to date. Bernard hopes it will yield more clues.
"The case has been an emotional roller coaster," Bernard said. "But there are no plans to send this to the cold case squad because we're still getting tips. All we need is that right person to come forward."
Anyone with information about the Precious Doe case should contact the Kansas City Police Department's Homicide Unit at 1-800-399-8517.
ABCNEWS' Pierre Thomas contributed to this report.