How Crimes Are Solved Years Later

In the first days, an investigator collects everything: a stain on fabric, a cigarette butt, a swab from a doorknob. Some of it is useless at the time - there's nothing to compare it with. A DNA profile doesn't name anyone on its own: it works only as a key to a list where that name already exists. No list means no answer, even when the evidence is literally in your hands.

At the same time, the backlog grows. The FBI has tracked the share of murders solved each year since about the mid-1960s, and it has dropped sharply: back then, about 9 out of 10 cases were cleared; in recent years, about half. The reasons are debated: witnesses speak to police less often, and detectives have heavier caseloads. But the result is the same - every year brings more boxes of evidence and unanswered cases.

Those boxes are the whole resource. In many countries, physical evidence in serious cases is kept for decades for exactly this reason: not because they expect a new theory within the case, but because they are waiting for something outside it - in a database, a lab, or someone's personal life. If the box is lost or the sample is damaged, there's nothing to wait for.

Since 2018, a technique called forensic genetic genealogy has been widely used. A profile from a crime scene is uploaded not to a police database, but to open services where people submit samples to search for ancestors. It doesn't need a match with the perpetrator herself or himself - third cousins are enough. A genealogist then builds the tree upward to shared great-great-grandparents and works back down, ruling out branches by age, sex, and place of residence. The result isn't proof, but a short list of people whose DNA should be collected directly.

That is how, in April 2018, a suspect was arrested in a series of assaults and murders in California in the 1970s-80s - the case had been considered hopeless for more than forty years, and 2 years later he pleaded guilty. It wasn't him who appeared in the database, but his distant relatives. The method doesn't always work: it needs a usable sample and enough relatives in open databases, and these are much more common among people of European ancestry - so in many cases, it simply stays silent.