Sometimes a crime-scene DNA profile returns no match in the national database — the offender simply isn't in it. But a partial match to someone who is can open a new door: that person may be a relative of the offender.
The logic
Relatives share more DNA than strangers, so a close family member's profile will partially resemble the offender's. Familial searching deliberately looks for these near-matches in the database. Investigators then:
- identify the close relative who partially matches,
- use conventional enquiry and genealogy to find their relatives,
- focus on a plausible suspect, and
- confirm with a direct DNA comparison.
The database match is only a lead; the case is proven by the direct test.
Not the same as genetic genealogy
Familial searching uses the STR profiles already in a police database. It's different from forensic genetic genealogy, which converts crime-scene DNA into a SNP profile and uploads it to consumer-style ancestry sites to find distant relatives. Both trace suspects through relatives, but they use different data and databases.
Why it's controversial
The power comes with genuine concerns: it effectively places the relatives of everyone in the database under a form of genetic surveillance, and studies have noted it can disproportionately affect communities already over-represented in databases. Many jurisdictions therefore restrict familial searching to serious crimes and require specific authorisation.