Getting a warrant for DNA

Question

In the US, if the police want to get a warrant to force someone to give a DNA sample in a murder investigation, is it always required that they must already have a suspect DNA sample to compare it to (i.e. collected from the scene, or believed to be connected with the crime somehow)?

I'll give a concrete example: suppose there's a murder, and the police don't have a DNA sample from the scene that they think could be the suspect. But they might get one some day - technology is always improving and there are clothes, objects etc. that might be enough, years from now. But it's speculative. Now suppose someone is named as a person of interest. Is it possible for investigators to get a warrant for that persons DNA? Or would the fact that there's currently nothing to compare it to make it impossible to get the warrant?

(I know that in some places they could take DNA if they arrest the person, but in this case I'm just asking about getting a warrant for DNA or "fluid samples" or whatever the term is. Let's say they don't have enough to arrest the person.)

Answer 1

In re Grand Jury Witness G.B. addresses the underlying legal principles. In that case, GB, the victim of a stabbing (not a suspect), appealed a court order for a warrant to take a DNA sample. A relevant federal rule is Rule 41(b), that

A warrant may be issued ... to search for and seize any (1) property that constitutes evidence of the commission of a criminal offense; or (2) contraband, the fruits of crime, or things otherwise criminally possessed; or (3) property designed or intended for use or which is or has been used as the means of committing a criminal offense; or (4) person for whose arrest there is probable cause, or who is unlawfully restrained.

The government maintained that the DNA sample is "evidence of the commission of a criminal offense". The court finds that the bar that must be clear is "cause to believe that the evidence sought will aid in a particular apprehension or conviction", and "evidence of the commission of a criminal offense" includes within its scope "evidence that might lead to other evidence".

The rule is not limited to the situation where the evidence sought would definitively inculpate or exculpate a person – it simply has to have some bearing on the crime in question. So the government does not need to argue "if we get this evidence and the test is positive, we will definitely get a conviction". Fishing is included in the scope of rule (41).