What Are Cold Cases and How Are They Investigated?

Cold cases are criminal investigations that remain unsolved after all initial leads have run out, leaving them inactive but not closed. The case file moves from an active detective’s desk into storage, where it waits for something to change: a new witness, a fresh forensic technique, a tip, or a reviewing investigator who spots what the original team missed. FBI Uniform Crime Report data shows more than 350,000 homicides went unsolved between 1965 and 2024, and that figure doesn’t include the unsolved sexual assaults, missing persons cases, and unidentified remains that make the true backlog much larger.

What Makes a Case Go Cold

There is no universal clock that triggers the “cold” label. Some agencies apply it after a year without progress; others wait longer. The common thread is that active investigation has stalled — detectives have run out of witnesses to interview, evidence to test, and suspects to pursue.

Cases rarely go cold for a single reason. They stall through a combination of factors that compound over time:

Weak physical evidence. Some scenes yield little biological material, and what was collected may have been handled with methods that wouldn’t meet today’s standards.
Witness problems. Witnesses may not exist, may refuse to cooperate, or may provide unreliable accounts. In gang violence and domestic abuse cases, fear of retaliation keeps people silent.
Technological limits. Crimes committed before modern DNA analysis, digital surveillance, and automated fingerprint systems had fewer tools available at the time. A hair sample from 1978 might have been collected but sat untestable for decades.
Institutional turnover. The original detective retires or transfers, and institutional knowledge about the case walks out the door with them. Successor investigators inherit a file they didn’t build.
Resource constraints. Cold case work competes with active caseloads for funding, lab time, and detective hours. The National Institute of Justice has noted that cold cases are an underserved category because agencies lack sufficient personnel, funding, and lab resources.¹
Lost or destroyed evidence. Evidence rooms flood, fires destroy stored materials, and in some cases from the 1960s and 1970s, investigators who believed a case had been fully worked simply discarded physical evidence to free up storage space.²

Why Cold Cases Can Still Be Prosecuted

Cold case work is worth doing because the law stays open. Murder carries no statute of limitations in any U.S. state, so a killer can be charged regardless of how many decades have passed.³ Federal law mirrors this: under 18 U.S.C. § 3281, any offense punishable by death can be prosecuted at any time.⁴ Solve a 1985 murder in 2026, and the suspect still faces charges.

Sexual assault cases are more complicated. At least 14 states have eliminated criminal statutes of limitations for certain sex crimes, but many others retain windows that can range from a few years to several decades.⁵ A cold sexual assault in one state may be prosecutable while an identical case next door is time-barred.

Who Works Cold Cases

Cold case investigation happens at every level of law enforcement, but the structure varies enormously by agency size and resources. Major city, county, and state departments sometimes staff full-time cold case units. Medium-sized agencies more commonly assign cold case work to a forensic services unit or a detective who handles it alongside other duties. Smaller departments often rely on regional homicide task forces to revisit unsolved cases.⁶

At the federal level, the Department of Justice runs the Cold Case Initiative, launched in 2006 to investigate racially motivated murders from the civil rights era under the authority of the Emmett Till Unsolved Civil Rights Crime Act. A dedicated cold case prosecutor in the Civil Rights Division coordinates the work.⁷

Two national databases tie local and federal work together. The National Missing and Unidentified Persons System, known as NamUs, lets law enforcement store, share, and compare case information including dental records and fingerprint cards, and it cross-references its missing persons records against its unidentified persons records. NamUs also provides free DNA analysis, forensic anthropology, fingerprint examination, and forensic odontology to law enforcement. As of mid-2025, the system had helped resolve more than 59,000 missing person cases and more than 8,300 unidentified person cases.⁸⁹ The FBI’s Combined DNA Index System (CODIS) does something different, storing DNA profiles from convicted offenders, arrestees, and crime scenes so that labs across the country can compare an unknown sample against known profiles. A CODIS “hit” can identify a suspect instantly or link multiple unsolved crimes to the same person.

Federal programs also target specific categories of backlog. The National Sexual Assault Kit Initiative exists because hundreds of thousands of sexual assault evidence kits sat untested in police storage for years, each one potentially carrying DNA that could link a perpetrator to a crime.¹⁰

How a Cold Case Gets Reopened

A case can come off the shelf for several reasons: a witness comes forward, a deathbed confession surfaces, a new forensic technique becomes available, or an investigator reviewing old files spots something the original team missed. The reopening typically involves a ground-up re-examination of the whole file: police reports, witness statements, autopsy results, and any stored physical evidence.

Fresh eyes matter more than most people realize. A new investigator may notice that the original team never followed up on a particular lead, or that a witness statement contradicts the physical evidence in a way nobody flagged at the time. The NIJ has put it bluntly: there’s a reason cases weren’t solved the first time around. Some are genuinely hard. Some were mishandled.²

Public attention plays a role too. Community tips, media coverage, and in recent years true-crime podcasts and documentaries have generated leads that put cold cases back on investigators’ desks.

The DNA Revolution

No single factor has done more to revive cold cases than advances in DNA technology. Samples once considered too small, too degraded, or too old to test can now yield usable profiles. The National Institute of Justice has specifically highlighted that biological evidence previously thought unsuitable for analysis may now produce results.¹¹

Touch DNA is one of the more recent developments. When a person handles an object, they leave behind skin cells containing their DNA. Modern extraction techniques can recover usable genetic material from a doorknob, a piece of clothing, or a weapon handle — surfaces that would have yielded nothing a decade ago. The amount is often vanishingly small, but improved amplification lets labs work with these low-template samples.

The process usually starts with uploading a crime scene profile into CODIS. If that search produces no match, investigators now have another option that didn’t exist before 2018.

Investigative Genetic Genealogy

Forensic investigative genetic genealogy has been the single biggest change in cold case work this decade. The technique came to public attention in 2018 when investigators identified the Golden State Killer, a serial murderer and rapist who had eluded capture for over 40 years, by uploading crime scene DNA to a public genealogy database and tracing his family tree through distant relatives.

The process works in stages. Investigators develop a DNA profile from crime scene evidence and upload it to a public genealogy database. The search identifies partial matches with people who share enough DNA to be distant relatives of the unknown suspect. Genealogists then build family trees from public records, connecting those relatives through common ancestors until the pool narrows to a small number of possible suspects. Traditional investigative work follows: surveillance, comparison with witness descriptions, and covert DNA collection.

The Department of Justice has issued an interim policy governing when and how federal agencies and DOJ-funded investigations can use the technique. The rules are strict. CODIS must have already been searched without success, other reasonable investigative leads must have been pursued, and a prosecutor must agree that genetic genealogy is a necessary and appropriate step. No one can be arrested based solely on a genetic association; standard DNA typing must confirm that the suspect’s profile matches the crime scene evidence before an arrest occurs.¹²

The policy also addresses privacy. Investigators must generally obtain informed consent from third parties before collecting reference DNA samples. If asking would compromise the investigation, they need prosecutor approval to collect a sample covertly, and a search warrant is required before a lab can analyze it.¹²

Why Solving Isn’t the Same as Prosecuting

Identifying a suspect is only half the battle. Prosecuting a case that’s 20 or 40 years old brings its own problems, and this is where many solved cold cases hit a wall.

Witnesses age, move, forget details, or die. A witness who saw the suspect’s car in 1990 may be unable to testify in 2026, and the original statement, while admissible in some circumstances, lacks the weight of live testimony. Physical evidence may have degraded despite proper storage, or critical items may have been lost during moves between evidence rooms. In some documented cases, evidence was intentionally discarded decades ago by investigators who believed the case had been fully worked.²

Defense attorneys in cold case prosecutions frequently argue that the long delay prejudiced their client’s ability to mount a defense: alibi witnesses have died, records have been destroyed, memories have faded. The absence of a statute of limitations for murder keeps the charges legally valid, but juries may still struggle with the passage of time. Prosecutors often lean heavily on forensic evidence to compensate.

Even when the suspect is identified and the evidence is strong, the person may already be dead. The case is technically solved, but no prosecution is possible. This outcome is common in civil rights-era cold cases, where suspects were sometimes known to investigators for decades but never charged.¹³

What Families Can Do

If a loved one’s case has gone cold, you’re not powerless. Contact the investigating agency and ask to speak with whoever currently holds the file. Ask whether DNA evidence was collected and whether it has been uploaded to CODIS. Ask whether the case has been reviewed in light of current forensic capabilities. Agencies vary in responsiveness, but persistent, polite follow-up keeps a case visible.

For missing persons cases, you can check NamUs directly. It’s publicly accessible and lets family members enter information that may help match a missing person with unidentified remains found elsewhere in the country.⁸ For civil rights-era crimes, the DOJ accepts tips at ColdCase.CivilRights@usdoj.gov.⁷

1
Office of Justice Programs. Cold Case Investigation Units and Advances in Investigative Techniques
2
National Institute of Justice. Applying Modern Investigation Methods to Solve Cold Cases
3
Congress.gov. Statute of Limitation in Federal Criminal Cases: An Overview
4
Office of the Law Revision Counsel. 18 USC 3281 – Capital Offenses
5
Federal Bureau of Investigation. Statutes of Limitation in Sexual Assault Cases
6
National Police Foundation. What Are Cold Cases and How Are They Investigated?
7
U.S. Department of Justice. Cold Case Initiative
8
National Institute of Justice. National Missing and Unidentified Persons System
9
National Institute of Justice. NamUs Statistics
10
Bureau of Justice Assistance. National Sexual Assault Kit Initiative (SAKI)
11
National Institute of Justice. Cold Case Investigations
12
Department of Justice. United States Department of Justice Interim Policy: Forensic Genetic Genealogical DNA Analysis and Searching
13
Department of Justice. Civil Rights-Era Cold Cases