What are the three ways the police use DNA traces in investigations, and how do they differ?

Question

Accepted Answer

Confirming a suspect (narrow context), searching databases for a known offender (extended context), and building a profile of an unknown offender (open context). The wider the context, the weaker the evidence and the greater the ethical risk.

* The three DNA contexts: each wider ring involves more people who are not suspected of anything *

Use
Steps
Strength

1. Confirm a suspect (narrow context)
DNA from the crime scene is compared with DNA taken from the suspect.
Very high evidential value.

2. Find a "known" offender (extended context)
The crime-scene DNA is matched against DNA stored in databases, including ones collected for other purposes (zweckfremd).
High value if there is a hit.

3. Find an "unknown" offender (open context)
A "phantom profile" of the likely offender's traits is built from the DNA, databases are searched for relatives, and a public dragnet search is run with the virtual profile.
Only probabilistic; many innocent people come under suspicion.

The ethical problems grow from step to step. Step 1 concerns only the suspect, while steps 2 and 3 pull in people who are not suspected of anything: those whose DNA was stored for another reason, their relatives, and everyone who happens to share the predicted traits.
Go deeper:

Wikipedia: DNA profiling — how profiles are built, matched and stored in databases.

Use	Steps	Strength
1. Confirm a suspect (narrow context)	DNA from the crime scene is compared with DNA taken from the suspect.	Very high evidential value.
2. Find a "known" offender (extended context)	The crime-scene DNA is matched against DNA stored in databases, including ones collected for other purposes (zweckfremd).	High value if there is a hit.
3. Find an "unknown" offender (open context)	A "phantom profile" of the likely offender's traits is built from the DNA, databases are searched for relatives, and a public dragnet search is run with the virtual profile.	Only probabilistic; many innocent people come under suspicion.