Contrary to the portrayals in popular culture, solving a cold case doesn’t happen that often.

Even with investigative methods made possible by technology such as DNA analysis, computer-generated age-progression images and connected law-enforcement databases, the clearance rates for cold cases remain low.

Nationally, about 1 in 5 cold cases investigated is ever cleared. One in 20 cases results in an arrest, and only one in 100 cases results in a conviction, according to a 2011 RAND Corp. study.

Complicating investigations of old cases are the ages of witnesses who might die before new evidence nudges an investigation forward. In other cases, the RAND researchers found, witnesses who were willing to testify early on subsequently get cold feet, or perpetrators can’t be located for prosecution because they are either dead or imprisoned.

The study surveyed more than 1,000 law enforcement organizations and found that dedicated cold case units are relatively rare. Only 20 percent of agencies even have a protocol for initiating a cold case investigation, 10 percent have dedicated cold case investigators, and only 7 percent had cold case units.

In the United States, cold cases involving unsolved homicides or missing persons aren’t tracked by any centralized law enforcement database or group.

In Maine, advocates have pushed since 2001 for the creation of a cold case squad with dedicated investigators. Despite a petition this year before the end of the legislative session that garnered more than 2,500 signatures online, the Legislature let die a bill that would have created a three-person cold case unit.

The state has more than 120 cases involving unsolved homicides or missing persons dating to 1953. The status of the cases or how many have been solved because of new techniques wasn’t clear. A Maine state police spokesman did not respond to a call for comment Friday afternoon.

Sheryl McCollum, who has been investigating cold cases for 15 years and now is an investigator for the Pine Lake Police Department cold case squad in Pine Lake, Georgia, said cold case investigations differ greatly from investigations involving more recent crimes.

Although the passage of time is usually a hindrance, McCollum said the passage of years or decades sometimes can free people to talk about things that were too sensitive for them when the crime was fresh.

And unlike most police investigations, cold case investigations are often undertaken by semi-professionals or amateurs who have little to no formal training, she said, including students of criminal justice. McCollum runs one such program in cooperation with Auburn University-Montgomery, Faulkner University and Bauder College, where students take on a new cold case each year.

McCollum and her students start at the crime scene and work their way through case details, re-examine facts and attempt to find new witnesses and evidence.

The news media, she said, can be a powerful force in cold case investigations, more so than when crimes are fresh, because added attention years later can expose the case to people who might not have been aware of it at the time the crime occurred.

Other times, police have strong suspicions who is guilty but must wait for evidence to substantiate a charge.

“A lot of times, the person who is guilty, their name is already in the file,” McCollum said. “They were interviewed by police. What you know and what you can prove is not the same thing.”

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