GENOMICS

The Terms for a Search

How a Law Enforcement Analysis of DNA Evidence Unfolds

See also: Feature: DNA Confidential: State Law Enforcement Policies for Genetic Databases Lack Transparency
Interactive Map: State Policies for DNA Crime Databases Vary Widely

State crime labs can collect and analyze DNA evidence, comparing results to profiles stored in the FBI’s Combined DNA Index System. Here’s how the process works in most states:

1. A violent crime occurs: for example, a sexual assault or murder. These crimes are the ones where a perpetrator is most likely to leave behind DNA.
2. The police investigate. They find, for example, hairs, blood, or semen.
3. Police send the sample to a DNA lab for analysis.
4. The DNA lab analyzes the sample. An initial search, looking for an exact match, reveals no match.
5. Later, during a routine database search, one or more partial matches show up as hits. For instance, it may be that at least one of these hits shares at least one allele at each of the available loci with the crime scene profile. Each locus is a point along an individual’s DNA where scientists look for identifying variants in the genes that make up the genetic code. Each DNA locus in a CODIS profile is made up of two alleles. This kind of match excludes the offender whose CODIS profile provides the match because that individual’s DNA is demonstrably different from the crime scene sample. But a partial match can implicate an offender’s close genetic relatives as possible perpetrators of a crime because they, like the crime scene sample, share some but not all of the examined loci with the individual whose CODIS profile provided the partial match.
6. An analyst performs additional testing.
7. If additional testing is concordant with a familial match, the name of the offender whose database profile provided the partial match may be released from the lab to law enforcement.
8. An investigation ensues. Will the new lead pan out? (Probably not.)

States with established partial match reporting policies interpose a range of additional requirements at steps 6 through 8. California has an explicit policy for how to handle “partial” matches and “familial” searches—each imposes several requirements for analysis before a new name can be released to police for further investigation.

First steps in DNA analysis for any California investigation:

1. A violent crime occurs: for example, a sexual assault or murder. These crimes are the ones where a perpetrator is most likely to leave behind DNA.
2. The police investigate. They find, for example, hairs, blood, or semen.
3. Police send the sample to a DNA lab for analysis.
4. The DNA lab analyzes the sample. An initial search, looking for an exact match, reveals no match.

For a “partial” match uncovered in California:

1. Later, during a routine database search, one or more partial matches show up as hits. For instance, it may be that at least one of these hits shares at least 15 of the possible 26 alleles at the 13 CODIS loci.
2. DOJ confirms that case is unsolved, the case investigator attests that all other investigative leads have been exhausted and requests analysis of the partial match(es), and the police and prosecutor commit to investigate the case if a name is released.
3. An analyst checks that the crime scene sample is single-source to ensure there were no mixtures that could cause contamination of the profile.
4. The analyst performs additional genetic testing, checking for identical Y-STR profiles. This is a profile of the Y chromosome, the male sex chromosome. All patrilineal males should have identical (or nearly identical) Y-STR profiles.
5. A DOJ committee reviews the evidence and finds no reason not to release this information.
6. The name of the offender whose database profile provided the partial match is released.
7. An investigation ensues. Will the new lead pan out? (Probably not.)

For a “familial” search in California:

1. No other investigative leads pan out.
2. A written request is sent to the Chief of the Bureau of Forensic Services describing the case, and attesting that all other investigative leads have been exhausted. The police and prosecutor commit to investigate the case if a name is released.
3. An analyst conducts a moderate- or low-stringency search designed to turn up partial matches. Some of the resulting hits share at least 15 alleles with the crime scene sample.
4. The analyst checks that the crime scene sample is single-source.
5. The analyst performs Y-STR analysis.
6. A DOJ committee reviews the evidence and finds no reason not to release this information.
7. The name of the offender whose database profile provided the partial match is released.
8. An investigation ensues. Will the new lead pan out? (Probably not.)

Natalie Ram is a Greenwall Fellow in Bioethics and Health Policy at Johns Hopkins and Georgetown Universities. She served as a summer research fellow at the Center for American progress in Summer 2009.

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