In working a cold case, it is very helpful to have records about the case. There are many sources of information, with varying degrees of access.
Case files are governed by state laws regarding access to public records. All states have some sort of freedom of information or public records law. Records requests are sometimes routinely denied on the grounds that a case is "open," but that might not be what the law says. Depending on the state, the agency might have to provide some records even for open cases. It might have to prove it had recently worked the case, or that prosecution is imminent. It might be required to redact personal information (for example, social security numbers) or withhold sensitive photographs, for example, rather than deny the request entirely on privacy grounds. Most states have a mechanism for appealing denials of records requests. These open records commissions are well versed in the law and will order appropriate disclosures. Over time, we will add individual state pages with links or templates for requesting public records. In the meantime, if you contact us we can ask a volunteer to walk you through it. Note: you may have to pay for records. If the case is large, the cost can run to hundreds of dollars. You should ask for a fee waiver because disclosure of the records is not for personal gain, or you might consider a GoFundMe or similar fundraiser to help pay for the records.
The FBI handles federal crimes, and sometimes becomes involved in local cases upon request. We often submit FOIA requests to see if the FBI has any information on a local case. (We usually request information on the victim and potential suspects, but remember that FOIA requests must show that the subject is dead or, if not, that the requester is otherwise entitled to the information.) Local law enforcement agencies often submitted evidence to the FBI for forensic testing. In cases where a serial killer was suspected, FBI files sometimes contain information on potential victims who were ruled out.
Contemporaneous news accounts are important to review, but keep in mind that they are often incomplete or even wrong. For example, police might feed the media false information in order to mislead a suspect. Law enforcement might confirm alibis or shift their focus without the new developments being reported by the media. Newspapers.com is a user-friendly source for news articles. (Note: For many newspapers, modern years require a Plus subscription.) NewspaperArchives.com is less user friendly but has a wide range of papers. GenealogyBank.com has a smaller range but extensive obituaries (many of which are also available at FamilySearch.org).
Some prison and jail records, including mug shots, are public records. Internal jail/prison records, such as the names of cellmates, are usually protected, but at a minimum you can get a timeline of each time a person was received or discharged from the facility. That can be quite helpful in determining if the person could have committed a crime at a particular time.
News articles often name the detectives working the case. In our experience, many law enforcement kept copies of records at home, including after retirement. (In one case, we were given 10 boxes of material from a storage unit.) They also sometimes had strong opinions or suspicions that they are happy to share now that they have the freedom of being a private citizen. Note: Some law enforcement are more comfortable discussing the case with a professional, such as an investigator or cold case organization.
Attorneys often remember interesting clients or cases, and may have kept case material. The photograph illustrating this section is of 3 boxes that a criminal defense attorney kept on an unsolved case where parts of the police file had been tossed. (Note: the Cold Case Coalition has attorney volunteers who can perform privilege reviews for attorneys to ensure that confidential materials are not disclosed.) Some judges also remember particular cases. They are less likely to have file material, but may be willing to share their memories or impressions.
Active and retired journalists may be a good source of information, particularly if there is a pre-existing connection. Some journalists have kept their notepads for decades. They may also remember information they were not permitted to publish or suspected but were unable to confirm.
The good news is that most court records are open to the public and copies or access are regularly provided upon request, sometimes without charge. (Notable exceptions include matters involving divorces and juveniles.) The bad news is that, before scanning became so cheap and easy, courts often threw away court files after a certain retention period without microfilming or scanning them first.
Many states have statutorily required archives or public record repositories. These archives may be a gold mine for cold case researchers. For example, some courts transferred old court cases to archives instead of destroying them. Some law enforcement agencies transfer closed case files for posterity (remember, a closed case might have information on the same suspect or witnesses as in your cold case).
Libraries (especially universities) often accept "papers" of potential historical value. For example, in one case we found transcripts of unpublished interviews with a suspect by an author whose papers were donated to a library at his death. In another, a mayor's archival papers included boxes of material about unsolved murders in his city. It's worth checking.